Aquinas’ Ways

Learning Objectives

Explain Aquinas’ premises and conclusions for his first, second and third ways.

Demonstrate how the three ways prove the existence of the unmoved, uncaused, necessary God.

Use Aquinas’ examples to illustrate his argument.

Key Terms

• Actuality – When something is in the state of doing something, e.g. fire is actually hot.

• Potentiality – When something has the power to be in another state, e.g. wood has the potential to be hot.

• Pure Act – The idea of something that has no potential to change but is instead pure and perfect actuality.

• God – Aquinas ended each Way by saying that the Unmoved Mover or Uncaused Case etc. was what we knew to be God.

• Infinity – The notion of no beginning and no end; this is a mathematical concept but is often proposed when considering creation.

• Infinite regression – The belief that the universe was not created at some fixed point but rather always existed.

• Sufficient reason – The principle that all things need a total reason to explain them.

Who was Thomas Aquinas?

• Thomas Aquinas was a 13th Century Benedictine Monk who translated much of Plato and Aristotle’s writings into Latin.

• In doing so he absorbed many of their ideas with his own and so established a great deal of Christian theology that we known today.

Who was Thomas Aquinas?

Aquinas wrote the summa theologica in which he presented his Five Ways of proving God.

Aquinas presented these as a posteriori arguments as they require experience and observation to work.

The First Way – Observing Motion

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion.

Motion is observable, therefore, anything that is based on this observation is a posteriori.

The First Way – One moving another

Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act.

Things are put into motion by other things so long as they have the potential to be moved.

The First Way – Actuality and Potentiality

For motion is nothing else than the reduction of something from potentiality to actuality.

This is taken straight from Aristotle.

Everything is in one actual state with the potential of being in anther state.

Motion is when you change from one state into another.

The First Way – The Mover

But nothing can be reduced from potentiality to actuality, except by something in a state of actuality.

Things can only be moved by things that are in some actual state already.

The First Way – Fire and Wood

Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it.

Use Aquinas’ example of fire and wood. The actually hot fire reduces the wood from being potentially hot to being actually hot.

The First Way – Actually and potentially

Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold.

A thing is actually one thing and potentially another.

The First Way – What moved it

It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must need to be put in motion by another, and that by another again.

A thing is moved by something else ongoing.

The First Way – Infinite Regression

But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand.

Without a first there would be no movement at all.

The First Way – Conclusion

Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

God is what we call the First Mover.

The Second Way – Observing Cause

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes.

A posteriori we can observe cause and effect.

The Second Way – Self-Cause

There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible.

Like with motion, a thing cannot cause itself.

The Second Way – Infinite Regression

Now in efficient causes it is not possible to go on to infinity, because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false.

The Second Way – Conclusion

Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.

There must be a necessary first cause to start all causes.

The Third Way – Possibility and Necessity

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be.

Some things are contingent and some are necessary.

The Third Way – Contingency

But it is impossible for these always to exist, for that which is possible not to be at some time is not. Therefore, if everything is possible not to be, then at one time there could have been nothing in existence.

Things that are contingent at one point did not exist. So all things did not exist at some point.

The Third Way – Something else

Now if this were true, even now there would be nothing in existence, because that which does not exist only begins to exist by something already existing.

There must be something necessary.

The Third Way – Contingent relying in necessary

But every necessary thing either has its necessity caused by another, or not. Now it is impossible to go on to infinity in necessary things which have their necessity caused by another

All contingent things require something necessary.

This cannot go on infinitely.

The Third Way – Conclusion

Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.

There must be something that is purely necessary –God.

Hume’s Challenges

Hume’s Criticisms

Many of David Hume’s (18th C) arguments against the cosmological argument are copied from William of Ockham’s rejection of Aquinas’s Ways.

This includes the criticism of assuming a connection between cause and effect since this cannot itself be experienced.

Hume’s Criticisms: 1. Like causes

Like causes resemble like effects.

Finite effects will have finite causes.

It would be more reasonable to assume that the world was created by male and female gods than to postulate the Christian God.

Hume’s Criticisms: 2. Experience

We have no experience of universes being made.

We know about causes within the universe but this does not entitle us to move to a cause of the universe as a whole.

Bertrand Russell said you cannot say that because a man has a mother mankind has a mother.

Hume’s Criticisms: 3. Logic

No proposition about existence can be logically necessary.

It is always perfectly possible to make an opposite statement to any existential proposition.

Like with the Ontological Arguemnt, you can’t go from de dicto to de re.

Hume’s Criticisms: 4. Necessity

The words ‘necessary being’ have no consistent meaning.

Any being that is said to exist could not exist.

‘All existential propositions are synthetic.’

Hume’s Criticisms: 5. The Universe

If by necessary being we mean imperishable being, then the universe itself may be necessary.

If things in the universe are contingent, then why can we not see the universe itself as necessary?

Bertrand Russell said that the universe might well be seen as a “brute fact”.

Hume’s Criticisms: 6. Infinite Regression

An infinite series is indeed possible, take mathematics.

As energy and matter are interchangeable, then why cannot it be seen that the universe is equivalent to energy which is eternal?

If an infinite series is possible then there is no need for a sufficient reason.

Hume’s Criticisms: 7. Causation

There is no way to prove causality.

You cannot prove that one event brings about another in any causal way.

When you hail a bus, you stop the bus, but you are not actually causing the bus to stop. It just appears that way.

Causation is just appearance.

To sum up

Like causes resemble like effects: we should not look to a creator unlike the creation.

We have no experience of universes being made: we cannot postulate what caused it.

No proposition about existence can be logically necessary: you can always postulate the opposite.

All existential propositions are synthetic: it means nothing to speak of necessary beings.

The universe is fact itself: if necessary means imperishable, then the universe may be necessary.

Infinite regression: why not appeal to infinite regression and reject “Sufficient Reason”.

You cannot establish causality.