Cosmological Argument – Third Way CA1(v)

The third way of the Thomist Cosmological Argument is a little different from the first and second ways and a bit more difficult to understand.

Aquinas begins with the observation that:

‘We find in nature things that are possible to be and not to be.’

These are sometimes called ‘possibles’.  Before going any further let us deal with this word ‘possibles’.  The word ‘possibles’ is plural noun, it is Aquinas’ name for a particular group of things.  One of these things would therefore be called a possible.  Don’t confuse this is the adjective ‘possible’, which describes something that might happen.  The words are closely related, but you must focus on the noun ‘possible’, the name for an object with a particular characteristic.

And what is this characteristic?  What is the definition of a possible?  Aquinas tells us that a possible is something that “can both be and not be”, that is, a possible can both exist and not exist, though obviously not at the same time.  So possible can come into existence and go out of existence.  For example. you are a possible.  There was a time when you did not exist, you came into existence and you will sometime stop existing, resulting in another time when you don’t exist.

There was a time when this blog did not exist, but now it does and one day, it will be deleted and cease existing.

This means that at some point in the past, each possible did not exist.  There was a time when you did not exist, this blog did not exist, the Sixth Form did not exist, Haslingden did not exist.

It is therefore a point of logic that if there was a time when each possible did not exist, there was a time when no possibles existed.  This can be a difficult point to understand.

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Imagine that there have only ever been three possibles line existence – my grandfather, me and my daughter.  My grandfather lived from 1929 to 2008.  I have lived from 1977 to now and my daughter lived from 2006 to now.  So you can see from this timeline, that at one point all three things existed, at some points, two things existed and at some point, only one thing existed. Therefore it is logical to assert that some point, none of these possibles existed.

Here Aquinas identifies the problem with this claim.  If at some point, no possible existed, then how did they begin to exist, because as we have seen in the second way, things cannot bring themselves into existence.  Since things can only be brought into existence by something already in existence, then there must have been something there to bring the possibles into existence.  But notice, this thing cannot have been a possible, since at this point, no possibles existed.

If this thing isn’t a possible, what is it?  A possible can both be and not-be, so therefore this thing must be something that cannot not-be.

If a possible can exist and can not exist, then something that is not a possible must not be able to not exist; it cannot not exist.

Therefore we call this thing that cannot not exist a necessary being, a being that must exist, and Aquinas says that this necessary being is God.


Modern versions of this argument use the word contingent rather than possibles.  A contingent being is, like a possible, a being that can be and can not-be, though contingent really means ‘depends on’.

In this instance, a contingent being depends on another thing for its existence, so depending on that other, these contingent beings either exist or do not exist.

If we use ourselves again as an example, we see that our existence is contingent, that is, our existence depended on our parents and if our parents had not produced us, we would not exist.

So in this argument, logic tells us that all contingent beings depend on something else for their existence, that is, there must be a non-contingent being on which everything else depends for its existence.  This non-contingent being does not, therefore, depend on anything else for its existence, so it must always exist.  It is the opposite of a contingent being, a necessary being, a being that cannot not exist, a being that must exist.  This being, the argument says, is God.


P1     Some things in this world are possibles – they can both be and not be.

P2     At some point, each possible lacks being.

P3     What lacks being can only begin to be by something which has being.

P4     Since all possibles have the possibility of not being, then at some point all possibles lacked being.

C1     There must be a necessary being, one that cannot not be.

C2     This is God.

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