Summary: Meditations on First Philosophy (page 4)

In the Fifth Meditation, Descartes presents a new proof of the existence of God.

As he demonstrated in the Second Meditation, we have a clear and distinct idea of extension, shape, motion, and duration — what Locke would later term primary qualities. And everything I know clearly and distinctly is true ¹.

Likewise, I have a clear and distinct idea of God as perfect, and therefore as existing, since that which does not exist cannot be perfect. This clear and distinct idea of God as existing therefore proves his existence.

Indeed, it is impossible for me to conceive of God as not existing: I manifestly find that existence can no more be separated from the essence of God [...] than from the idea of a mountain the idea of a valley ². Thus, from this alone — that I can conceive of God only as existing — it follows that existence is inseparable from him, and hence that he truly exists ³.

Having deduced the self from the cogito and then God from the self, Descartes now seeks to deduce the world from God. If God exists as a perfect being, he cannot deceive us — which guarantees the reality of the external world and opens the way to genuine knowledge of it:

The certainty and truth of all science depend on the knowledge of the true God alone, so that before I knew him, I could know no other thing perfectly. Now that I know him, I have the means of acquiring perfect knowledge concerning an infinity of things — not only those that are in him, but also those that belong to corporeal nature ⁴.

The question now arises: which things in the world can be regarded as real? All of them? We have already seen, however, that the senses deceive us.

This is the question Descartes addresses in the Sixth Meditation.

Descartes begins by distinguishing between imagination and intellect through his famous example of the chiliagon.

I can imagine a triangle, but not a chiliagon (a figure with 1,000 sides). Any mental image I form of it will be indistinguishable from that of a myriagon (a figure with 10,000 sides). Yet I can still conceive of a chiliagon and grasp what it means: a figure with 1,000 sides.

Imagination serves as a link between body and mind, a certain application of the faculty of knowledge to the body, which is intimately present to it and therefore exists ⁵.

Thus, imagination itself serves as proof of the existence of my body. The intellect further confirms this existence, since I have a clear and distinct idea of my body as an extended thing that does not think ⁶.

As for other corporeal things, they must exist — otherwise God would be a deceiver. However, they do not necessarily resemble what the senses reveal to us. What is certain are the qualities that Locke would later call primary qualities: extension, duration, and so on. At least it must be confessed that [...] all things, generally speaking, which are included in the object of speculative geometry, are truly found there ⁷. Other qualities — such as colours and light — are more doubtful.

Thus, the hyperbolic doubt at the root of these meditations can now be set aside as vain and unnecessary, giving way to a more precise and refined form of doubt.

Descartes is now led to examine more closely the relationship between mind and body, whose existence he has just demonstrated.

He compares this relationship to that of a pilot at the helm of his ship — a metaphor that has endured.

He ultimately concludes that the body is like a machine:

I consider the body of man to be a machine so built and composed of bones, nerves, and muscles that, even if it contained no spirit, it would still move in the same ways it does now, [...] solely by the disposition of its organs ⁸.

Descartes would go on to develop this fundamental doctrine in The Passions of the Soul.

¹ p.167
² p.169
³ p.170
⁴ p.177
⁵ p.180
⁶ p.190
⁷ p.193
⁸ p.201