De Trinitate VIII, 6, 9.
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With reason we say that we know what a soul is since we have a soul. We haven’t seen the soul with the eyes of the body nor have we perceived its likeness either in a general or specific way among the many things we have seen but rather we know what the soul is because we have one. Really nothing is more intimately known nor better perceived than that through which we perceive everything else, that is, the soul itself.