John Locke

“ 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 ”

“ General propositions that are certain concern not existence. Hitherto we have only considered the essences of things ; which being only abstract ideas, and thereby removed in our thoughts from particular existence, (that being the proper operation of the mind, in abstraction, to consider an idea under no other existence but what it has in the understanding,) gives us no knowledge of real existence at all. ”

“ Where, by the way, we may take notice, that universal propositions of whose truth or falsehood we can have certain knowledge concern not existence : and further, that all particular affirmations or negations that would not be certain if they were made general, are only concerning existence ”