Aquinas presents a different way of arguing for the existence of God. It was a feature of Anselm’s argument that its premises did not seem to rely on any facts about how the world is; the proof proceeded solely on the basis of the fact that we can imagine a certain sort of being. Aquinas’s arguments, which are often called versions of the cosmological argument, begin with certain very general facts about the universe, and argue from these to the existence of God.
He presents five such arguments, which we will discuss in turn.
Aquinas’s first argument is based on the idea that the fact that things change shows that God must exist. The argument appears to run as follows:
| 1. | Whenever something changes, it moves from potentially having a property to actually having the property. |
| 2. | Whenever something moves from potentially having a property to actually having that property, this must have been brought about by something which actually has the property. |
| 3. | Whenever something changes, this change must have been brought about by something which actually has the property which is acquired by the object that changes. (follows from 1,2) |
| 4. | Nothing can simultaneously both potentially and actually have a certain property. |
| 5. | Whenever something changes, this change must have been brought about by something else. (follows from 3,4) |
| 6. | Some things change. |
| 7. | The chain connecting things which change and things which initiate the changes cannot be infinite. |
| C. | There is a first mover, which initiates change but is not itself changed. (follows from 5,6,7) |
A worry about this argument: it does not seem to show that there is a unique first mover.
A second worry about this argument: it seems to imply that the first mover would have to have contradictory properties, e.g. being both hot and cold. A possible source of this difficulty in the constraints put upon change.
A second, formally similar argument relies on general facts about objects coming into existence (rather than objects changing, or acquiring new properties). This argument may be understood as follows:
| 1. | Everything which has come to exist has been caused to come to exist. |
| 2. | Nothing which has come to exist can be the cause of its own existence. |
| 3. | Everything which has come to exist is caused to exist by something other than itself. (follows from 1,2) |
| 4. | It is impossible for a chain of causes of this kind to go on to infinity. |
| C. | There must be a first cause, which causes other things to come into existence but did not itself come into existence. (follows from 3,4) |
Can a chain of causes go on to infinity?
You might want to respond to this argument (and to some of those that follow) by granting the conclusion, but denying that it shows anything about God: perhaps there was a unique first mover, but why should we think that this first mover had any of the properties that we traditionally ascribe to God? Aquinas was aware of this worry, and spent some of the sections following the ‘five ways’ trying to resolve it.
A very different kind of argument, which does not rely on the impossibility of infinite chains of any kind, is based on the distinction between beings which exist necessarily and those which exist only contingently. The intuitive distinction between necessity and contingency.
| 1. | There exist now contingent beings (beings which could fail to exist). |
| 2. | Whatever could fail to exist, at some time does not exist. |
| 3. | If all beings are contingent, then at some time nothing existed. (follows from 2) |
| 4. | Whatever begins to exist is caused to exist by something which already exists. |
| 5. | So if at one time nothing existed, nothing exists now either. (follows from 4) |
| 6. | If all beings are contingent, nothing exists now. (follows from 3,5) |
| C. | Not all beings are contingent, and so there must exist at least one necessarily existing being, which is God. (follows from 1,6) |
Questions about the logical transition from (2) to (3).
(See Plantinga, God, Freedom, and Evil for more discussion.)
(We will not be discussing this argument.)
(This is actually a version of what is sometimes called ‘the argument from design’, which we will discuss in more depth when we read Paley and Hume in the next section of the course.)