It is claimed that Aristotle believed that all the materials of knowledge and all the concepts it involves are derived from experience. Aristotle believed that we think we have knowledge about something when we know its reason or cause. Knowledge of an object includes contextualizing that object in a logical proposition or a syllogism. An example of such a syllogism is: All mammals have hair. Humans are mammals. All humans have hair. Aristotle would rely upon the logic of syllogisms also to determine some thing's reason or cause. Something's cause is closely linked to its essence in Aristotle's writings. For instance, the essence of an axe is to cut wood. The essence of humans is much more complicated, although it could be said simplistically such as "to live a good life."
Aristotle's dilemma with epistemology was in trying to show how particular reasons (reasons meaning fundamental qualities of an object such as "to cut or structured to cut" for the axe), also informed us of universal facts. Thus, we generally know something when we know its particular reason for being. The question Aristotle attempts to answer but does not is whether it is the particular axe's function that gives it meaning or if it is some universal idea of "axeness" that gives it meaning. If it is a combination of the two, meaning we learn of a particular (via experience) and this leads us to a universal truth, the idea is that this universal truth exists independent of our thinking and Descartes would differ on this point.
Descartes' approach to knowledge was scientific and philosophical but more subjective. He introduced a first-person approach to epistemology. His famous "I think, therefore I am" indicates that he proves his existence himself. In doing so, he showed how knowledge derives from the mind. This is a more rationalist approach than an empirical one. Unlike Aristotle, Descartes did not believe that all our ideas derive from experience (the empirical doctrine). However, he did concede the possibility of innate or "a priori" (before experience) ideas which is similar to the notion of a universal truth.
Descartes sought to establish a unified, scientific epistemology. But in terms of contributions to epistemology and consciousness, he emphasized how our senses can deceive us and supposed that thought itself is the basis for deriving knowledge. Thus, he supposed the possibility that there is no external world and everything we experience is an illusion. Descartes exposed the limits of the individual consciousness and the senses and this helped to define the scope of epistemology.
Juxtaposing the two, Aristotle focused more on the reason for something's existence. We derive this reason with experience and logic. Descartes focused more on how to prove what exists at all. Descartes separated the mind and sensory experience. He thought that the mind is much more reliable than the senses in acquiring knowledge with certainty.