 Performance of this second task, in turn, entailed the third, the accuracy and verifiability of the knowledge which would constitute the contents of his updated philosophy. Verifiability depends on two interdependent factors for the person doing the verification: following a productive method and having the mental apparatus to employ that method and understand its results. The method Avicenna adopted already at the start of his career was logic, and the mental apparatus wherewith we know involved an understanding and study of the human, rational soul. Thus logic and the theory of the soul as the basis for epistemology are the two motors driving Avicenna’s philosophy. He wrote more, and more frequently, on these two subjects than on anything else.