In Happiness and the Art of Being, chapter 5, 'What is True Knowledge?', after the paragraph (on page 277 of the present e-book version) that ends, "Is it not clear, therefore, that the only true knowledge that we can attain is the clear knowledge of ourself as we really are, devoid of any superimposed adjuncts — that is, knowledge of ourself as our unadulterated and essential self-consciousness, 'I am', which is the absolute non-dual consciousness that knows only itself?" I will incorporate the following addition:
All objective knowledge involves a basic distinction between the subject, who is knowing, and the object, which is known. It also involves a third factor, the subject's act of knowing the object.
Because our knowledge of ourself involves only the inherently self-conscious subject, and no object, we know ourself just by being ourself, and we do so without the aid of any other thing. Because we are naturally self-conscious, we do not need to do anything in order to know ourself. Therefore unlike all our objective knowledge, our knowledge of ourself involves neither an object nor any act of knowing, and hence it is a perfectly non-dual knowledge.