“[I]n a sense Aquinas laid more stress than did the classical British empiricists on the part played by sense-perception in human cognition. For while not excluding introspection or reflection as a source of knowledge, he did not mention sense-perception and reflection as parallel sources of knowledge. He did not think that introspection or reflection is an initial source in the same sense in which sense-perception is. His point of view was that I become aware of my existence as a self through concrete acts of perceiving material things other than myself, inasmuch as I am concomitantly aware of these acts as mine. I do not enjoy a direct intuition of the self as such: I come to know myself only through acts directed towards things other than myself. I not only perceive a man, for example, but I am concomitantly aware that I perceive him, that the act of perception is my act. And this awareness involves the awareness of my existence as a self. ‘The soul is known by its acts. For a man perceives that he has a soul and lives and exists by the fact that he perceives that he senses and understands and performs other vital operations of this kind. . . . No one perceives that he understands except through the fact that he understands something, for to understand something is prior to understanding that one understands. And so the soul comes to the actual realization of its existence through the fact that it understands or perceives’ (De veritate, 10, 8).

“To prevent misunderstanding of this passage it should be added that Aquinas draws a distinction between my awareness of the existence of the self and my knowledge of the nature of the self. To know that I have a soul or that there is in me that by which I perceive, desire, and understand is one thing: to know the nature of the soul is another. For the latter knowledge deliberate reflection, ‘second’ reflection, is required; but the reflection by which one is aware of the self in a very general sense is not a deliberate reflection, and it is common to all human beings. It must not be confused therefore with philosophic reflection: it is automatic in the sense that I cannot perceive without an implicit awareness that I perceive. And the point is that my awareness that I perceive is dependent on my perceiving something. I can indeed reflect consciously and deliberately on my interior acts; but this presupposes a non-deliberate or automatic awareness of my outwardly-directed acts (of seeing, hearing, desiring, and so on) as mine. And this in turn presupposes the fundamental rôle of sense-experience or sense-perception. Aquinas believed that man does not consist of two juxtaposed substances, the operations of which are independent one of another, but that he is a unity, the soul being naturally united with a body. And because of the intimate union of soul and body the mind is naturally dependent on the senses for the acquisition of ideas and of knowledge.”

— from F. C. Copleston, Aquinas (Penguin Books, 1955)

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