I am unable, when I turn to myself, to recognize any of my faculties or my capacities. The inner sensation which I have of myself informs me that I am, that I think, that I will, that I have sensory awareness, that I suffer, and so on; but it provides me with no knowledge whatever of what I am — of the nature of my thought, my sensations, my passions, or my pain — or the mutual relations that obtain between all these things … I have no idea whatever of my soul.

Dialogues on Metaphysics (1688) Dialogue III