2. A threefold knowledge of existence.
But, leaving the nature of
propositions, and different ways of predication to be considered
more at large in another place, let us proceed now to inquire
concerning our knowledge of the existence of things, and how we come
by it. I say, then, that we have the knowledge of our own existence by
intuition; of the existence of God by demonstration; and of other
things by sensation.
3. Our knowledge of our own existence is intuitive.
As for our own
existence, we perceive it so plainly and so certainly, that it neither
needs nor is capable of any proof. For nothing can be more evident
to us than our own existence. I think, I reason, I feel pleasure and
pain: can any of these be more evident to me than my own existence? If
I doubt of all other things, that very doubt makes me perceive my
own existence, and will not suffer me to doubt of that. For if I
know I feel pain, it is evident I have as certain perception of my own
existence, as of the existence of the pain I feel: or if I know I
doubt, I have as certain perception of the existence of the thing
doubting, as of that thought which I call doubt. Experience then
convinces us, that we have an intuitive knowledge of our own
existence, and an internal infallible perception that we are. In every
act of sensation, reasoning, or thinking, we are conscious to
ourselves of our own being; and, in this matter, come not short of the
highest degree of certainty.