Prior knowledge in design: a dynamic

knowledge-based model of design and creativity

Rivka Oxman

Facuby of Architecture & Town Planning, Technion Israel Institute of Technology, Haifa, 3200 Israel

The incorporation of precedents into a present design situation by adaptation, restructuring and reformulation depends upon processes of typification and generalization. The function of prototypes as a characteristic form of generalized and structured knowledge in design is described. A precedent-based design model employing menmry-based reasoning is proposed. It is argued that ~ypological concepts can serve as a matching level between situation types and solution types. In such an approach both the organization of structured knowledge and the mechanisms of matching to prior knowledge such as cross-indexing and analogy appear to be of seminal importance. A memory-based reasoning process in routine, innovative and creative design is postulated. This is based on concepts of dynamic and episodic memory. Relevant work in the fields of memory organization, machine learning and analogical

reasoning is considered with respect to its significance to the field of knowledge-based design.

Keywords: knowledge-based design, design knowledge, memory-based reasoning, creativity

The objective of this paper is to investigate the implica- tions of cognitive concepts of dynamic structures and dynamic memory in design, in the phenomenon of creativity, and their potential for knowledge-based de- sign systems. In order to accomplish this, we first explicate the nature of design knowledge and the role of the structure and organization of design knowledge in memory. We introduce a multi-level structure of design knowledge from specific, context-dependent precedents to the high level knowledge of prototypes and concepts. Typification and generalization of knowledge from design experience are seen as the processes which develop this complex structure of design knowledge. These concepts provide a theoretical basis for a knowledge-based dynamic model of design. We then consider the important process of indexing to prior design knowledge and discuss the

relevance of analogical inference mechanisms. Finally, we consider some of the research questions of analogical reasoning in creative design.

The large and well-developed body of research in artificial intelligence in the area of memory organization, primarily associated with Schank 1-3, provides a theor- etical basis for this work. The important related area of study, machine learning, provides the concepts for dealing with conceptualization, generalization, indexing and analogical matching of knowledge in design. Further- more, it offers insights into learning during the process of design, a phenomenon characteristic of the exploratory nature of reasoning which is integral to design. The significance of this paper lies in its implicit aims to extend our understanding of design through artificial intelligence concepts and methodologies and to contri-

Vol 11 No 1 January 1990 0142-694x/90/01017-12 $03.00 O 1990 Butterworth & Co (Publishers) Ltd 17

bute to our ability to accommodate knowledge-based creative design beyond routine design.

The specific design domain in which these ideas are explored is that of architectural design. One of the interesting phenomena of the last two decades has been the effort, both on the part of researchers and designers, to reformulate and to rigorously formalize what might be thought of as architectural knowledge 4. It is the existence of this work which makes the present effort to incorpor- ate well-formulated knowledge into knowledge-based design systems possible to consider, as well as opportune.

DESIGN KNOWLEDGE

Prior knowledge in design

Observations on research in design methodology by Cross 5 have shown that the various design methods shared a common view of design as a cyclic process which contains an analytical phase of problem definition and formulation, a synthesis phase of design solution and an analytical phase of evaluation. One of the characterizing aspects in the emergence of a theory of design methods has been the belief that logic and rational procedure were in themselves the foundations of design. Method trans- cended content in the attempt to forge a modern approach distinct from the precedent-based traditions of past design. The common approach was that 'The logical nature of the act of designing is largely independent of the thing designed '6. In their enthusiasm to distance themselves from past design traditions based upon preferred models, modern design theoreticians not only refocused emphasis upon procedure, but tended to denigrate the recourse to tradition and stylistic prece- dents. These assumptions were questioned by other researchers who were concerned with explicating cogni- tive processes of design 7, and cognitive structures of design knowledge s based upon past experience and accepted tradition. The term tradition is employed here less in the stylistic sense than in the epistemological sense; tradi- tions, such as artistic traditions, formulate the knowledge of precedents.

Current design theoretical literature contains frequent references to design as a form of knowledge 9'1. Prior knowledge in a particular structured formulation of under- lying types such as concepts, prototypes and precedents is an intrinsic ingredient of design. Schon 9 focuses on similar underlying types such as functional building types, references, spatial gestalt and experiential archety- pes, and describes their roles in skilled designing. It is an assumption of our work that design is, in fact, a dynamic process of adaptation and transformation of the knowledge of prior experiences in order to accommodate them to the contingencies of the present.

In such an interpretation, design is seen as founded upon cognitive processes which are involved in the classification and adaptation of design knowledge. De- signers can adjust an initial encoding of their world experience to reflect the necessities of newly encountered

situations. In order to understand this complex process, we require a theory of knowledge structures and adaptation processes which can help to explain how knowledge in memory is adapted in order to create new knowledge, and how these adaptations of experience have a role in design generation and creativity. In this respect, design has an analogous relationship to learning, since design may be conceived of as a form of learning in which the existence of prior conceptualizations are the threshold for the formation of new knowledge. Modell- ing the way in which design experience is interpreted, classified and stored as knowledge may provide the basis for the implementation of these ideas in knowledge-based design systems.

The role of structured knowledge in design

Designers utilize their past experience in various ways. Experienced designers are able to solve complex prob- lems through cognitive processes which appear to economize search and adaptation by, among other things, the characterization of design problems H, prob- lem abstraction and generalization 7. The embodiment of the experience of the designer in the form of design knowledge is dependent upon the structure and organiza- tion of knowledge in memory. The designer's memory provides the format for design learning and for the storage of newly acquired knowledge. It is likely that among these cognitive processes are the typification and generalization of design experience. The generalization of knowledge contributes to a hierarchical structure of levels of knowledge. It is this structure which provides the medium for accessing prior knowledge in a creative way.

The experienced designer is insightful because of his access to generic knowledge within a richly structured world. The organization of knowledge in memory is a reflection of our generalizations, both personal and cultural. Part of the role of a tradition, or of a paradigm in the cultural sense 12, is the provision of a structure of knowledge. Design provides a rich field for the explora- tion of knowledge structures as well as mechanisms of retrieval and transformation which employ and modify that structure. The reasoning processes which occur in the recall and restructuring of knowledge are among the foundations of design; they may also provide a basis for the explanation of creativity.

Concepts from the theory of dynamic memory 2 are relevant to the application of these ideas to design, since we may assume that the way in which the designer applies knowledge in memory depends, at least in part, upon memory organization. Knowledge structures are organized in such a way that they support the process of exploratory thinking which is characteristic of design. Searching through these structures in order to select relevant knowledge can be regarded as part of the creative process. Researchers in both memory theory and machine learning have investigated the mechanisms of knowledge retrieval is. These mechanisms offer us an important interpretation of reasoning processes in de-

18 DESIGN STUDIES

sign, and one which has utility in helping to explain design. In terms of relevance to knowledge-based design, these ideas offer promise for reasoning mechanisms that support innovative and creative design.

Typification and generalization in accessing prior design knowledge and knowledge structuring

In the theory of memory organization and knowledge formation one of the key concepts is generalization. In daily life, generalization appears to be controlled by sets of abstraction levels similar to those postulated by Schank 1'2. Generalization is a process of accessing and classifying prior knowledge by abstracting the specifics of a newly encountered object, or situation, in order to match with prior knowledge in memory. Such an interpretation of generalization implies that one powerful medium of memory organization, as well as memory indexing and accessing, is the abstracting process of generalization. Typification is a form of generalization, which involves making typical in order to accommodate experience within some schema, or classification of concepts. Typification is, therefore, controlled by cate- gories of higher level abstractions which are part of the structure of knowledge in memory.

As knowledge becomes highly typified and abstracted, it becomes relatively domain-independent in the form of design concepts and principles which may be relevant to various design domains as a form of general design knowledge. One such form of generalization in design is the ability to typify situations, constraints and goals during the early, conceptual stages of design. In current work constraints have been used to model design. Gross et al. TM use constraints as knowledge representation schema in design. Oxman and Gero ~s use both con- straints and goals to describe a process of design related to knowledge, in which the designer typifies the design problem in order to fit it to the closest solution type in memory. Typification and abstraction continues through levels of abstraction until some match is achieved. Abstraction levels are levels in a continuum from the specific, contextual knowledge to the context- independent knowledge of higher level concepts. These are levels within a hierarchical, multi-level structure of design knowledge. Transition to a higher abstraction level of design goals can continue until a match between goal and conceptual solution is made. It is then modified in order to fit the required profile of requirements. The movement upwards through abstraction levels characte- rizes typification.

The initialization of design occurs by referring to significant typifications based on design experience. We shall sketch two aspects of the typification phase: specification typification, including constraints and goals; and formal typification, including artefacts and opera- tions.

Intrinsic constraints as situation typification

Typification can be considered the abstraction and classification of salient aspects of precedents in terms of both situations and solutions. Situation typification is a classic category of memory organization in which classi- fication is in terms of recurrent types of situations. In design, it can also include the particular physical constraints which are usually imposed by these recurrent types of situations. Situation typification is an important and well-formulated type of classification in architecture. Some common classes of typifications of situation derive from site geometry, orientation, topography and access conditions. For example, the typification of local con- straints in architecture includes aspects related to the context such as the physical characteristics of the site. Factors such as useable percentages of site area or site geometries are generalized as cases which are recognized from prior knowledge and are associated with paradigms of approach to site planning. Constraints which are intrinsic to the context are termed intrinsic constraints. These are reinterpreted as extrinsic constraints which are, in fact, typified design goals. These goals enable potential matching to typified solution types.

Extrinsic constraints as goals typification

The typification of goals and constraints and their propagation in design is a process which continues throughout design. Goal typification is, in itself, a reflection of the designer's experience, since the infer- ence of higher level goals from the initial statement of lower level constraints is a reflection of experience and insight. Inference in initial goal typification in architecture is already based on knowledge such as formal architectural knowledge. Formal types may also be consi- dered as goals in the typification of the design brief and in its expression employing morphological, or functional, types. This process of typification generalizes the charac- teristics of the brief on the basis of preferred solution prototypes. Formal prototypes may override site con- straints. We would refer to this as a formalist approach. For example, a court house would normally not be considered as a prototype associated with sloped sites. Given a formal typification-- house with private interior space-- an override might take place. Eventually a unique sub-type may result which is subsequently stored in memory as a solution indexed as novel in its differences from the standard script of sloped site solutions.

The process of the restatement of goals as confignra- tive types may be as follows

initial intrinsic constraints: open site extrinsic goal typification: privacy, identity redefined goal typification: introspective, enclosed

The process of goal typification in any domain may include higher level knowledge such as goal-themes which

Vol 11 No 1 January 1990 19

are part of the content of knowledge of a design tradition, an ideological movement, or a style. An example in architectural planning of such thematic goal statements might be the concepts open, or flexible, as a potential matching for formal typifications of architectural solu- tion types. A design tradition may also, in itself, formulate universally valid goals such as efficiency or mmtmlzauon.

Goal typification is a form of reasoning which is essential to related subjects such as creativity. Typifica- tion may also apply in the formulation of design goals at an appropriate level of abstraction. A new goal category is seen in terms of a known goal cateogry and designers often reinterpret goal specifications as they design. One of the capabilities of generalization of the experienced designer is the ability to appropriately explicate the implied goals of a particular design prototype with which he may be working. For example, a new concept of marketing, the 'drug-store', is a typological innovation motivated by goal redefinition due to the principle of the gathering of people as a prerequiste to retailing.

Design precedents typification

One of the most significant forms of typification in design is that of formal typifications of descriptions of design. Such typification establishes the classes of formal types above that of explicit precedents. This would include, for example, syntactic classes of solution type as higher level knowledge than functional types or particu- lar precedents. Figure 1 refers to such a structure of formal typological knowledge. Such formal and typolo- gical knowledge constitutes one of the most important classes of design knowledge and is particularly relevant knowledge in design generation. Formal grammars are an example, and the Palladian Grammar 16 is one of the most well known formulations.

The assumption underlying this interpretation of the

cognitive processes is that design knowledge is, among other things, knowledge of typification through abstrac- tion levels. A high level of typified and generalized domain-dependent knowledge makes possible the cor- relation of situation types to solution types by matching at a level of typological knowledge and formal abstrac- tions. Figure 2 illustrates this.

NEW FROM KNOWN: THE PARADOX OF PRECEDENT-BASED CREATIVITY

A basic assumption of this work is that current design is based upon prior design experiences which have been abstracted and encoded in a structured form in memory. This entails an apparent paradox in that the knowledge of the precedent is, by definition, of the past. How can it be used not only to explain, but to generate, the new? Does a theory of typified knowledge contradict the possibility of creativity?

How are new solutions derived from knowledge of past solutions? A fundamental factor would appear to be the classification of prior solutions in memory as abstract and generalized knowledge stored in a structure of abstrac- tion levels. The high level abstraction of knowledge can, in itself, contribute to the creative application of prior experience in design. An example would be the mechan- ism of cueing to relevant knowledge through a novel reformulation of the problem in order to make a creative connection with precedents. Metaphoric thinking is one such mechanism for novel problem interpretation. Furthermore, from high level abstractions of solution types, numerous derivations are theoretically possible within the class of that type. The historical distinction between iconic source and generic type has been frequently employed to explain the usage of high level abstractions as a source of architectural knowledge, types being capable of diverse design solutions from one generic typological source.

Formal prototypes: [ ~l [ I architectural non-contextual ~ ~ , ~

Functional prototypes: [ ~ q ~ [ ] l ] functional context

Precedents : highly contextual

Figure I. Precedents typification through the generalization of high level concepts

20 DESIGN STUDIES

Constraints

Goals

Typological formal prototypes:

Situation typif ication

Typological formal prototypes:

Functional prototypes:

Precedents :

Solution typif ication

Figure 2. Typological concepts as a matching level between situation types and solution types

The prototype as a medium for the formal definition of structured design knowledge

One promising structuring schema for design knowledge is the prototype. Prototypes can be defined according to various types of content such as functional or formal prototypes. They offer a convenient basis for the organization of various classes of complex knowledge. Diverse categories of knowledge such as the functional and configurational can be integrated in prototypes. Our knowledge of prototypes, though domain-dependent, tends towards the generalized. We use the terms archetype and type to refer to the generalized qualities of specific generic structures of knowledge in design. The prototype provides a medium for representing this complex structure of knowledge. I t is a formalized framework for the organization and structured representation of the complex high level knowledge of a typological category in a design domain. This includes various types of knowledge such as the salient characteristics and vari- ables of the type, processes of generation, classes and variables of sub-types, the forms of refinement and modification, the interpretation and control of these processes. Furthermore, the prototype also contains membership in higher level conceptual schema which are

important for their role in creative design. With respect to knowledge-based design systems, the use

of the prototype, as a knowledge structure, provides an effective basis for the organization of design knowledge. The prototype can be regarded as one of the fundamental structures of design knowledge which can be employed to encode knowledge. A prototype according to Gero 17 may be defined as follows

P = (Dpg, I, V, K)

where Dpg can be a parametrized design description or parametrized design generator; I represents a set of typifications based on interpretations of conditions, requirements or goals; Visa set of design elements which are the basis for design description; and K is the knowledge relating design descriptions and its interpreta- tions. In a routine design situation a prototype is selected and specialized to the task at hand by providing values to parameters. But design usually requires some form of accommodation of the general description of the proto- type to the specifics of the new situation. I t is in its adaptability, as well as in the descriptive strength of the prototype, that its power as a medium of knowledge-based design resides.

Vol 11 No 1 January 1990 21

The prototype in the continuum of design creativity

The use of the prototype in design can be classified 17 into three processes of design generation: prototype refine- ment, prototype adaptation and new prototype genera- tion. Routine design, or prototype refinement, is the lowest level of prototype-based design in which a prototype is selected and specialized to the task at hand by providing values to parameters.

Routine design can be described as

D = t (Dpg, I)

where D is a design description as a result of transforma- tion processes, t, which are acting on Dpg, the design prototype.

Prototype refmement utilizes a successive process of transformation in order to generate design descriptions in knowledge-based design systems. Current knowledge- based design systems fall into this category ls'19. A concept of a generative prototype for a refinement process has been proposed and implemented employing design prototypes as a general design shell s'2. Its use was demonstrated in the architectural domain of a dwelling plan type.

In adaptive design, or prototype adaptation, the pro- totype is selected but requires modification before refinement. Transformation processes can modify know- ledge, including vocabulary elements and interpreta- tions, incorporating them into modified design descrip- tions.

D -- t(Dpg', I')

In original design, or prototype generation, the prototype is not available and needs to be generated from existing knowledge. Knowledge-based design systems have been, until now, largely restricted to routine design. Compiled knowledge of a prototype does not allow the designer to step outside the current design situation. This is post- ulated as being a major characteristic of design in non-routine design. We suggest that this is achieved by reference to higher level concepts which make connections between precedents and prototypes and assist in the creation of a new prototype out of the existing ones.

The role of high level concepts in non-routine design

In non-routine design we are interested in how new, or modified, vocabularly elements and/or goals can be found and incorporated. Goal-constraint propagation is the term applied to the phenomenon of the redefinition of goals and constraints on the basis of a current design situation. It is the redefinition of goals and constraints through the process of typification according to some structured representation of past experience which pro- vides the prototype match, even in non-routine design.

Our assumption is that creative derivations are based on particular structures of knowledge which share high-level concepts which may be common to several prototypes. A resultant restructuring and reorganization of knowledge can occur which, in effect, generates a new prototype. This formulation appears to be germane to the explana- tion of the exploratory processes of creativity.

In matching to a prototypical design, or part of a prototypical design, the designer would go to a concep- tual level, in order to determine the implications of a required adaptation. The representation of design know- ledge as prototypes includes the formalization of such modifiers, or operations of transformation. Adaptation includes procedures for the transformation of a typolo- gical class. In the prototype, the two are combined, with the prototypical solution carrying its own rules of transformation integrating the interpreters and modi- fiers.

Wittkower 2~ has described the role of the typological and adaptive in the villa designs of Palladio. From a first match to preference, an ideal type (double axis symmetry in a centralized 'nine-square' plan) is localized and made specific through a particular vocabularly of modifications such as providing local emphasis upon one axis to accommodate sites with changes of level, while still preserving the integrity of the main monumental axis of approach.

There are also general modifying operations which are associated with relatively domain-independent general knowledge. The characteristics of the museum or the house as a building functional prototype become purely organizational and con figurative at a higher level of abstraction. The structure of typological knowledge would appear to include configurative knowledge which evolves from specific typologies (the Palladian Villas), to general typologies (the centralized plan), to general configurative knowledge (centralized configurations). This continuum from the specific precedent towards domain-independent general knowledge enables the de- rivation of modifying operations and adaptive strategies. Figure 3 elaborates the ideas in Figure 1 with known architectural precedents.

There is potential significance in the dynamic inter- relationship between the formal schema of a prototype and goal-constraint redefinition. The example illustrates goal generalization through context typification and processes of adaptation of a generative prototype. The writings and work of Kahn 22 provide an explicit example of redefinition of design goals by generalization in order to be able to match to an organizational prototype. It is interesting to observe the sequential transformation of the generic type of spatial organization as it is accommo- dated, and adapted, to context. Contextual information involves the identification of constraints which derive from the local or the contingent. Localization is essential- ly the cause for the adaptation of the ideal solution in a conceptual state to be adjusted to the realities of place and time. In the theory of the architect Kahn 22 there is a distinction between form and design. Form is a Platonic pre-form, an archetype of solution class in a pure state;

22 DESIGN STUDIES

o

.v

>, C h u r c h t y p e

C e n t r a l i z e d t y p e

x.x.x.,....~.. .......

E

F i r s t U n i t a r i a n C h u r c h K a h n 1959-1967

L i b r a r y t y p e

E x e t e r L i b r a r y K a h n 1967-1972

V i l l a t y p e

I I

DIeD I I

Vi l l a Ro tonda Pa l lad io 1566-1571

Figure 3. Precedents, functional types, high level typological concepts

design is the accommodation of form to the contingent. Thus constraints are the sources of modifications and adaptations in the general class of solutions. In a memory-based system, interpretation can include gener- alizing to fit the vocabulary of precedents in memory or any simplification which makes match, or fitting, possible.

A DYNAMIC MODEL OF DESIGN

One of the implications of this work is that there appears to be an integral relationship between the organizational structure of design knowledge through prototypes and high level concepts and the processes of design. It is this organizational structure which is relevant to the repre- sentation of knowledge in knowledge-based design sys- tems. In the following sections, we discuss the organiza- tion of design knowledge in memory and the role of episodic indexing in creative design. We summarize with discussion of a memory-based approach and its relevance to creativity in design in knowledge-based systems.

Semantic and episodic design memory

Design knowledge derives from the building of gener- alizations as high level concepts on the basis of episodic experience. It is the structure of design knowledge which is germane to creativity in design, including storing knowledge gained through episodic design experience, and accessing that knowledge in current design. Experi- enced designers traverse the space between the episodic and the generic with some ease. Typification is the concept which we have introduced to describe the process of indexing an episodic event to a class of events which are stored in the designer's memory. These

concepts have implications for structuring of design knowledge in memory.

Memory organization theory deals with the rela- tionship between the structure of knowledge in memory, and the implications of the structuring for the enhancement of the matching processes. Theories of semantic memory 23 depict memory organization as a series of concepts related to one another. Once the concepts are estab- lished, they are not changed even after being used. Episodic long-term memory provides the conceptual orga- nization through which the events related to these concepts can be found. It is this organizational structure of memory which provides the ability to generalize about episodes from particular experience 13. Episodic memory therefore has a significant role in design. Events in episodic memory can be regarded as precedents in design. Designers accommodate the known to the new. This involves classification and integration with what they already know. Memory is in a constant process of dynamic reorganization as the important and relevant is rendered accessible. Memory must be appropriately organized to accommodate various operative mechanisms of matching and retrieval. Matching may be through complex processes of abstraction, typification and asso- ciation.

The role of episodic memory and precedents indexing in design

Design precedents can be stored and retrieved in episodic memory under preferred design indices. Designs would be organized in memory by what they have in common as generalized indices, and discriminated within these classes by their differences. Creating indices for designs is enhanced when a rigorous, formalized structure already exists. The organization of memory can reflect

DESIGN STUDIES 23

the way precedents and prototypes are associated with higher level concepts. A design can be retrieved from memory by using these concepts as indices, thus permitting precise focus in search.

How is new information accommodated in a design memory? Classification is a significant cognitive process in design. The designer uses what he knows in order to process current design experience. The experienced designer categorizes his experience by generalization, which makes it available in processing new experiences; and, vice versa, new experience can contribute to the growth and adjustment of categories of knowledge. How prior experiences are categorized affects the accessibility of their knowledge in current situations. Memory is organized under conceptual generalizations based upon similarities. Kolodner's description of episodic memory organization 13 recommends that 'similar items in mem- ory should be organized around the same concepts according to their differences'. In this way items can be referenced in various ways including the possibility of cross-contextual indexing. Adding events to episodic mem- ory is accomplished by selecting appropriate indices and modifying them by differentiation in order to create new indices. Not all episodic events are stored directly in memory. An episodic event would be stored under some feature which discriminates it from the pattern type. Storage by class, by discrimination features, is an indexing mechanism which can contribute to the creation of new classes.

Design descriptions, as we have stated, are stored in memory according to what they have in common, their generic qualities; they are organized under such indices and discriminated by differences. Such an organizational schema contributes the ability to gain generalized know- ledge from particular precedents and episodes. The more general the description of features in an index, the more accessible the index. The generalization of the unique features stored in indices enhances matching and retriev- al in design. Memory growth, the acquisition of design experience, is that process of differentiation of knowledge through experience, which both particularizes existing conceptual categories and creates new ones. Thus, the storage of new designs, or new design experience, results not only in the expansion of memory, but also, potentially, in a dynamic process of memory reorganization.

We have suggested that the theory of episodic memory can be used to establish a dynamic memory organization in design. Some of the other key issues in retrieval from episodic memory are the significance of levels of abstrac- tion to indexing and to cross-contextual remindings.

Designs from different domains would be indexed at various levels of abstraction. For example, in a building domain, a court has a function associated with outdoor activities. At a higher level of abstraction, a courtyard may have a role as a separating element between spatial zones. The more abstract the description of objects and attributes, the more the possibility of retrieval of the same object in various domains. For example, the court as a separating space between zones can be retrieved as a typological device in various functional prototypes such

as a museum, a residence or a school. We also assume that the more general the description of designs with respect to abstraction levels, the deeper its hierarchy and richer its conceptual structure; this contributes to a higher capacity of the system to exhibit reasoning leading to creativity. Creativity may involve sophisticated mechanisms of reminding in which partial matching to solutions at one level can cue a match at a higher level of abstraction.

Designers may have personal styles of creating indices for reminding and retrieval. Experience in design from the point of view of memory theory is the particularization and relationships of indices. We assume that all design experience undergoes processes of typification in order to create indices for the storage, and ultimately for the retrieval, of design episodes; and that the way in which this occurs is a function of the form of classification and existing structure of the designer's memory.

Formal indices in architectural design

Comprehension of the designer's utilization of memory in the application and adaptation of designs will also help to understand how memory is applied in creative design. First of all, as we have already suggested, this is connected with the conceptual structure of design knowledge. Our ability to remember depends upon our ability to form this underlying conceptual representation. The experienced designer has a large repertoire of conceptual indices which are used for the purpose of, reminding and a well-developed technique for matching for relevance to various situations.

Remindings in design requires the recognition of similarities between current design situations and prior design experiences. The designer's experience provides the cues which index relevant solutions. Such cues may be strongly domain-dependent, but they may also be cues to the generic and conceptual knowledge of the particular domain. In architecture, cueing to solution types in the initial phase of design is conceptual in nature. Architectural concepts in this phase deal mainly with the formal qualities which reflect significant domain concepts. For example, in museum design, the key factors as interpreted from a functional point of view are the organization and character of displays, the relationship to the circulation system, and control. These are factors which define the prototype as a set of formal indices. These formal prototypes and key relationships of dominant variables form sub-classes of the prototype which are the typological variations. The museum as a building proto- type has relevance for all other functional prototypes in which similar formal indices are part of the typological definition. It is clear, therefore, that cross-contextual reminding is a significant phenomenon in design.

MEMORY-BASED STRATEGIES FOR MATCHING

Prototype-based creativity derives from the capability of

24 DESIGN STUDIES

conceptualization which elevates the designer from the level of the domain to the level of principle at which cross-contextual connections can be made and precedent modifiers inherited. This invokes certain strategies for generating novel designs in which a level of abstraction and cross-contextual remindings occurs. What occurs when an exact match does not exist, and how is matching made? Matching has an important role in knowledge- based design. The designer matches the situation to prototypes and conceptual schema. In the matching process the retrieval of similar designs and relevant precedents is according to the level of specificity of the index which is matched. The search for matching in memory is selective and focused on the relevant exam- ples. In the real world, there is probably no exact match. How can memory be searched for relevant matches without search being exhaustive? Among the strategies which can be applied in memory-based reasoning in design are partial matching and reconstructive reasoning; cross-contextual matching; explanation matching, and analo- gical reasoning.

Partial matching and reconstructive memory

One of the characteristics of human memory is the ability to cue reminding on the basis of partial cues. Reminding can be activated by few cues. This is a form of reminding somewhere between a conception of rote reminding in episodic memory, and reminding by the mechanism of reconstructive memory. Reconstructive memory may be understood as follows: 'when people try to retrieve information about particular episodes, they do not have the entire episode, immediate and available. They try to reconstruct the rest of the episode '13.

Partial matching to wholes is one approach. Part to whole description z4 involves constructing the description of a whole object by observing only selected parts of it. These parts may be described by diverse relevant attributes. The process of construction of the whole from a partial set of diverse attributes may result in unique combinations due to the reconstructive process. Thus reconstructive memory may, in itself, be viewed as one form of creative behaviour. It is a mechanism which may be used to overcome incomplete, or insufficient, know- ledge of a domain, and may help to produce new indices for retrieval of relevant designs from other contexts, or for the creating of new designs.

The establishment of matching criteria is an important topic in matching. Performance specification is a useful basis for matching criteria in design, but the application of such criteria is not a simple matter of match and retrieve, particularly when non-routine solutions are required. Dyer et al. 25 have pointed out some problems of matching and relevance. In matching to the perform- ance criteria of door operation, for example, bicycle mechanisms are irrelevant. On the other hand, matching to the gear mechanism of a bicycle might contribute to the development of an innovative opening mechanism for doors. Therefore, recall by specifications and within

contexts must be directed through various levels of abstraction. This example indicates the particular rele- vance of matching at high conceptual levels in creative design.

Partial matching relies on our ability to reconstruct the whole picture by pulling a fragment of the description. Triggering past designs is probably not by an explicit, or complete, description of a past design. Partial cueing may be through one characteristic of that design such as a goal, a constraint or certain explanations associated with the design. This bead can then help to pull the whole description of a past design through a reconstructive process.

Domain-free cross-contextual matching

Cross-contextual matching appears to be another of the characteristics of creativity 2. A rich body of high level concepts in design will support cross-contextual indexing and enhance the possibility of adapting by combining elements from solutions of diverse problem contexts. Such matchings appear to be dependent upon abstraction levels which function as the mechanisms which transcend domain boundaries. Knowledge expansion strengthens these links to the generic and the ability to make cross-contextual linkages. Designers may store this high-level generic knowledge in the pithy form of a metaphor, and the high level typification applies to a leading knowledge source in the domain, such as spatial configurations in architectural design. Le Corbusier's chambered nautilis metaphor for an 'endless museum' was adapted in various designs. Metaphorical reasoning is a form of reasoning in which there is a creative use of matching. Experience enhances the ability to generalize which is critical to the ability to make cross-contextual references. Pulling relevant memory from cross-contexts requires an organizational schema for knowledge which has some communalities of indices at a high level despite a diversity of contextual distinctions at lower levels.

Explanation matching

The understanding of how certain designs have de- veloped may be connected with the designer's ability to be creative. When we attempt to understand why something was designed, we search in our memory for a set of values or goals in order to explain it 3. This would support our main assumption that it is abstraction levels which both reflect and define the explanatory power of the representations in our memory.

Goal specifications at a generalized level may have similar motivations in diverse contexts and provide a potential basis for reminding in design. The 'why', indexed to the 'what' and the 'how', may serve as an index for the retrieval of a description of the prototype and its generative rules. An example of the process in architecture is

Vol 11 No 1 January 1990 25

index-goal: high density; match to high-rise urban form index-goal: communality; match to village form retrieve representations of." high-rise scheme + street organization scheme from rural form and integrate organizational prototypes

This is the phenonmenon which promulgated the high level of experimentation to develop new housing types in Europe for high density living during the 1950s. The specification was high density which cued a high-rise building type. Because an additional goal was social interaction, the type was criticized as alienating and inhibiting interaction. A new generalized goal was formulated as interaction between people and non- alienation. There was a period of cross-contextual reminding during which these goals were associated with village life and primitive societies which were admired for their social cohesiveness. Designers attempted to solve this problem by achieving village attributes at high density. One attribute of the village was the importance of in-between space and street life. The matching of the new goal of 'street organization' brought its own rules, and the prototype, high-rise housing structure, was adaptated accordingly. The matching of street into tower is one of the interesting stories of typological adaptation in post-war European architecture. The history is quite interesting, since there also existed a precedent which controlled the adaptation process until a new type emerged. This was Le Corbusier's Unite building at Marseilles, one of the first 'streets in the air'. Furth- ermore, the social attributes and adopted goal, interac- tions between people, was also inherited by other building lypes.

Analogical reasoning

Analogical reasoning may be one way to move across different contexts at the conceptual level, by referring to explanations and concepts. There are various strategies based on analogical reasoning which can be employed in matching procedures. Before reviewing some computa- tional studies of analogy and their relevance to design, a common terminology between these researchers will be introduced and will be adapted for design throughout the rest of this section. Analogy will be described as a matching between concepts of a source domain design and a target domain design 26. The analogy matching maps abstractions and descriptions from the source domain into a target domain. These mapped descriptions are analogical inferences and are verified by confirmatory schemes.

Carbonell embeds analogical reasoning within a tradi- tional problem solving framework 27'28. He has proposed two methods: transformational analogy and derivational analogy. Transformational analogy transforms a solution from a previous problem, the source domain, for the target domain. Derivational analogy applies a complete trace of a previously solved problem. It includes sub-

goals and alternative operators at each level with reasons for decisions. It evaluates not only the actual solution path, but also a record of why particular steps were taken and succeeded or failed. This information is necessary particularly in a complex domain like design. Carbonell's approach to analogical reasoning involves: identification of significant aspects which in design can be general goals, formal concepts, functional concepts, etc.; selection of relevant experience from organized memory; focusing on relevant knowledge to be transferred from past solution to current solution; transformation-modification which in the case of an existing design supports alternation such as the variation of an attribute, function, etc. Analogical reasoning can be employed to cue to similar prior situations by reviewing features of past experience. Dissimilarities between the current situation and situations in memory are then reviewed in order to achieve the closest match. A similarity matrix can be used to establish the mapping. It can employ context- sensitive expressions which are capable of identifying relevant similarities between two object or process descriptions.

Knowledge-based creative design: a memory- based approach to adapting, restructuring and reformulating design prototypes

Current work in the field of knowledge-based creative design is concerned with applying the techniques from artificial intelligence and cognitive science described above. An approach in the domain of engineering design 29 is based on the association of novelty with creativity by providing an analogical mechanism for introducing one or more design elements which are imported from a source domain to the target domain. Another approach is based on the retrieval of fragments of previous design episodes and their incorporation into the present design context 3. A further approach in the domain of mechanical design 25 is based upon designs being categorized on the level of class descriptions. In the EDISON system, each device is stored in memory under different class-descriptional attributes such as functional, typological or situational. Designs can be retrieved by focusing on any of the attributes. EDISON considers three general strategies for creating new designs: gener- alizations, analogies and transformations. All of these rely on memory organization, indexing and retrieval. At some point in the modification of a prototype, the performance is also transformed. With the transformation of object attributes, a transformation of performance is achieved which is more than a sub-class, since it cannot be accommodated within the similarity matrix. A new type has emerged through transformation. In EDISON, certain heuristics for creativity, such as variations of attribute scales, are proposed. One example of such a scalar variation in architecture is Frank Lloyd Wright's expansion of the ramp principle to become the organiza- tional concept of the Guggenheim Museum. Other forms of variations, such as variations of use and function, are also possible sources of design creativity.

26 DESIGN STUDIES

S U M M A R Y A N D C O N C L U S I O N S

This paper has attempted to provide a theoretical basis for memory-based reasoning in knowledge-based systems to support design creativity. Furthermore, some of the significant questions which are associated with the development of memory-based systems for non-routine design were identified. Until now in recent work in knowledge-based systems, domain knowledge was uni- que compiled knowledge and did not permit the designer to step outside this knowledge base. Memory-based reasoning and matching processes appear to permit the designer to step outside a domain base as he relates to an open world which consists of prototypes and episodic precedents. In the previous sections we have dealt with design knowledge, the significance of its structure, the indexing of past experience in memory, and some of the existing approaches for the use of matching mechanisms and analogical reasoning.

Within the model of non-routine design which we have proposed creativity is enhanced by the ability to general- ize to a high level of abstraction. The analogical processes, the operations of modification, and the actual mechanisms of activation constitute significant research topics in this field. Computational studies of these processes must elaborate them in the research framework of design computing. Design knowledge in different domains must be studied and formalized. As for the architectural domain, architectural design knowledge requires formulation as a research topic. Artificial intelligence techniques will provide the potential for a computable model for creative design only when domain knowledge will be understood and well formulated. Ultimately, it is our understanding of the complex and dynamic nature of knowledge structures which will enable the development of knowledge-based systems for design.

A C K N O W L E D G E M E N T

The author wishes to thank Professors John Gero and Robert Oxman for stimulating discussions.

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