[Essay II xxiii 22-25] It is easy enough to employ an...

[Essay II xxiii 22-25] It is easy enough to employ an empiricist version of Descarte's cogito ergo sum as a demonstration of my own existence, of course. Thinking of any sort, feeling pleasure and pain, even the act of doubting itself, are all experiences that carry with them a full assurance of my own existence as a thinking thing. [Essay IV ix 3] Notice, however, that Locke declined the further implications of the Cartesian inference to sum res cogitans.

Thinking, he argued, is merely an activity of the soul, not its essence; so the continued existence of an individual thinking thing does not necessarily entail its continuous consciousness. [Essay II i 10-16] Locke frequently expressed significant reservations about the demonstrability-if not the very truth-of Cartesian dualism.

Given our widespread ignorance of the inner constitution and operation of substances generally, Locke notoriously suggested that, for all we know, the power of thinking could be providentially superadded to an organic human body as easily as separate thinking and material substances could be combined. [Essay IV iii 6] But the ultimate origin of thinking itself is another matter.

The Existence of God According to Locke, the existence of God is an instance of demonstrable knowledge in any reasoning being. Since I know intuitively that I exist as a thinking thing, and since nothing can be made to exist except by something else which both exists and has powers at least equal to those of each of its creations, it follows that from all eternity there must have existed an all-powerful cogitative being.

[Essay IV x 3-6] This amounts to a variation on the Aristotelean / Thomistic cosmological argument for God's existence: there is an instance of thinking; every thinking thing proceeds from some other thinking thing; but there cannot be an infinite regress; so there must have been a first thinking thing, or God. What most interested Locke in this argument was its emphasis on thinking.

Willing though he was to contemplate the possibility that individual human beings are animal bodies that have the power to think, Locke insisted that mere matter-with its primary qualities of solidity, mass, and motion-could never on its own give rise to cogitative activity of any kind. Although it may not strictly be inert, matter is most definitely unthinking. [Essay IV x 9-10] Materialism can never account for the emergence of thought in a universe containing only senseless matter.