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. . . . I think, I reason, I feel pleasure and pain: can any of these be more evident to me than my own existence? . . . 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.
John Locke (1632 - 1704)
Source: Essay Concerning Human Understanding, 1690