Jason asked:

How does Descartes' theory "I think, therefore I am" prove the existence of God?

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"...from the mere fact that I exist, and the idea of a sovereignly perfect being, that is to say of God, is
in me, the existence of God is very clearly demonstrated.

Descartes, 'Meditation III'"

Descartes' famous argument, 'I think, therefore I am,' forms the basis from which all the other
arguments in the Meditationsproceed. In the first Meditation he raises the logical possibility that we
are being deceived by an evil demon, and as such we cannot be sure of even our most basic
assumptions, for the demon may trick us into believing that things are true, when in fact they are not.
We have made errors in the past or been mistaken about things that we thought we were most
certain of. Perhaps I am dreaming that I am writing this reply now and I'm not really sitting here in
front of my computer at all! In order to overcome the most radical sceptic who causes us to question
the existence of the external world, we need an indubitable starting point that can be used as the
foundation for further enquiry. Descartes' takes his argument, 'I think, therefore I am,' as that starting
point and attempts to move via a series of logical inferences from that proof to the conclusion that
God must exist.

Even if there is an evil demon attempting to deceive us about all our beliefs then there is at least one
truth that he cannot prevent us from knowing. If I am considering whether there is an external world,
or whether there is an evil demon or not, then there is an 'I' that is doing the thinking. Every thought
must have a thinker. So from the fact that I am thinking it is possible to infer that I exist. If I am in the
process of doubting what I can know then I must exist in order to do the doubting. Once one truth has
been established we can use this to ascertain a reliable method for further inquiry. Descartes
surmises that we can trust things that we conceive clearly and distinctly.Thus Descartes uses his first
principle, 'I think, therefore I am,' to derive a method which can be used in his attempts to prove the
existence of God.

In the third Meditation Descartes puts forward his first main argument for the existence of God. It can
be set out as follows:

1. I exist and have the idea of a God who is sovereign, eternal, infinite, unchangeable, all knowing, all
powerful and universal creator of all things.

2. I am an imperfect being.

3. Something which is more perfect, or that which in itself contains more reality, cannot be a
consequence and dependent on the less perfect.

4. Therefore the idea of God is too great for me to have been its cause (from 1. and 2.).

5. Only a perfect being could have given me the idea of God that he possesses.

6. So God must exist.

The argument rests on the claim that it is impossible for Descartes to have acquired the idea of God if
God does not exist. Descartes argues that as a finite being he wouldn't have the idea of an infinite
substance, such as God, unless the idea had been put into him by some substance that is truly
infinite. So as Descartes exists (derived from 'I think therefore I am') and he has an idea of God which
he could only have got from God himself, God must exist too.

However, the above argument has a fundamental weakness which is revealed if we consider the
move from point 5. to 6. above. Even if it is conceded that an imperfect being, such as a human
being, cannot have acquired the idea of God with his infinite attributes alone, the possibility of a being
with more power than us but less power than Godgiving us this idea is still left open. Descartes'
Cartesian demon would be a good example of a being that could perhaps give us the idea of God
even if God does not exist. Perhaps this is part of his grand plan of deception! Since there is another
means by which we could acquire the idea of God that does not entail the existence of God,
Descartes' argument here fails to establish its conclusion. Do we have to be acquainted with infinity in
order to have the concept of an infinite substance? Are allour ideas acquired through experience or
acquaintance? If not, then another flaw in Descartes' argument has been identified.

In the fifth Meditation Descartes offers another argument for the existence of God, yet again taking 'I
think, therefore I am' as the main premise from which the argument proceeds. This is well known as
the ontological argument for the existence of God.

1. I find the idea of God, a supremely perfect being,in me.

2. Existence is a perfection.

3. A supremely perfect being is perfect in every way by definition.

4. It is a contradiction to suppose that God does not exist, as this would involve him lacking a
particular perfection, namely existence. (from 1.—3.)

5. So God necessarily exists.

Descartes makes this point, stating that, "...existence can no more be separated from the essence of
God than the fact that the sum of its three angles is equal to two right angles can be separated from
the essence of a triangle or than the idea of a mountain can be separated from the idea of a valley."
So once again Descartes' argument moves from the fact that he exists and possesses the idea of
God, to the conclusion that God must exist. Has Descartes succeeded in proving the existence of
God this time? The ontological argument is discussed at length in Brain Davies book, Philosophy of
Religionoffers a convincing criticism of the ontological argument, which is discussed in Davies' book.
So can the ontological argument withstand these criticisms?

Samantha Solomons