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Logic and Persuasion

AGED 520V

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Why do scientists need to know logic and persuasion?

• Scientists are writers and must persuade their audience…– That their ideas have merit– That their research is credible

• Valid, reliable, believable, verifiable, transferable

– That their findings are interpreted correctly– That their conclusions are logical– That their recommendations are practical

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Aristotle’s Modes of Persuasion

• Logos– Appeals to an audience’s sense of logic

• Ethos– Appeals to an audience’s sense of right and wrong

• Pathos – Appeals that take advantage of an audience’s

propensity to let emotion guide decision-making

• Appeals to logic are the longest-lasting and generally the most effective in scientific communications

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Investigative and Argumentative Methods

• Deduction– An argument constructed with the intention

of establishing that the conclusion must be true if the premises are true.

• Induction– An argument that proposes a likely

conclusion based on what has been observed

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Deductive Logic• An argument is a set of propositions, one of which

(the conclusion) is supported by the rest (the premises)

• Propositions can be presented as declarative statements, expressed as fact, or at least as a claim

• A premise is a declarative statement, which should be a reason, expressed as a “given”

• A conclusion is a declarative statement, presented as a logical deduction according to the premises

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Deductive Logic, cont’d• The standard form is…

– Premise (Every college student owns a computer)– Premise (Catherine is a college student)– Conclusion (Catherine owns a computer)

• A clue that someone is presenting a premise is the use of certain introductory words/phrases:– since…, because…, given that…, the reason that…

• Keywords indicating conclusions include…– so…, thus…, therefore…, consequently, it can be concluded

that…, it follows that…

• Authors who choose to overtly demonstrate the logic of their argument use these words/phrases for clarity

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Sound Arguments

• A bad argument is still an argument:– “The stock market is in full decline, and the moon

will be full in two days. It follows that aliens are visiting the earth.

• Two conditions for a sound argument:– The argument must be valid

• The conclusion follows directly from the premises

– All premises must be true• Defining truth is difficult, but there are many truths that

humanity agrees upon• Appealing to an audience’s sense of truth or reality is the

key to being persuasive

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Deductive Logic in Empirical Studies

• Empirical studies in social science most often use hypothesis testing to logically deduce conclusions

• Hypotheses state expected relationships between two or more variables

• When inferential statistics are used, researchers commonly use a null hypothesis to disprove that no relationship exists– Studies are set up to test for no significant difference among

means– When the null hypothesis is rejected, a premise leading to a

conclusion has been created

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Deductive Logic in Empirical Studies, cont’d

• Example: – H0: No relationship exists between students’ time devoted to studying and final

exam score– Finding: A significant difference in final exam score existed between those who

studied for four hours and those who studied for 30 minutes– Conclusion: The findings led to rejection of the null hypothesis that no relationship

exists– Implication: Data show that a relationship exists between time devoted to studying

and final exam score– Recommendation: Students should spend more time studying if they want a better

final exam score

• The logic:– Premise 1: A significant difference in exam scores between students who study

four hours and students who study 30 minutes would indicate that students who study more can increase their exam scores

– Premise 2: Data show that a significant difference between exam scores exists.

– Conclusion: Therefore, students should study more to increase their exam scores.

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Deductive Logic in Empirical Studies, cont’d

• Example of the logic involved in inferring these findings to a larger population: – Premise 1: Students in the group observed who studied

more performed better on the final exam– Premise 2: Through random sampling, the students

observed were representative (have the same characteristics as college students all over the U.S.)

– Conclusion: College students all over the U.S. who study more perform better on the final exam

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Inductive Logic• Inductive logic is often the alternative that is used when

empirical studies and deduction are not appropriate• Conclusions focus on what is most likely and reasonable rather

than on what is empirically true• Validity and reliability give way to credibility, believability,

confirmability, etc.• Includes premises made through general observations

– Premise: Every swan that has ever been observed is white Conclusion: Therefore, all swans that will ever be observed will be white(There may be no way to prove that all swans in the future will always be white, but it is reasonable to make this conclusion based on what we know about the observed population)

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Inductive Logic, cont’d• Used most often in observational research

– Phenomenology– Case studies– Ethnographies– Clinical and naturalistic observations– Other qualitative investigations

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Inductive vs. Deductive

Specific observation (premises)

DEDUCTION

Generalization to other situations, groups, or

entire population

General observations or knowledge about

populations (premises)

INDUCTION

Application of general traits to specific

situations or future occurrences

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Inductive Logic