This paper analyses primarily De anima II, 5 to illustrate the Aristotelian understanding of knowledge as an act. The originality of Aristotle's proposal is evidenced in the contrast of his doctrine with those of the majority of the pre- Socratic philosophers who had a rather materialistic conception of knowledge. The doctrine of power and act allows Aristotle to explain knowledge in terms of an actualization or realization. With the addition of some key texts of Metaphysics the novelty and subtlety of his proposal may be evidenced.