Aristotle defines philosophy in terms of essence, saying that philosophy is "the science of the universal essence of that which is actual". Plato had defined it as the "science of the idea", meaning by idea what we should call the unconditional basis of phenomena. Both pupil and master regard philosophy as concerned with the universal; Aristotle, however, finds the universal in particular things, and called it the essence of things, while Plato finds that the universal exists apart from particular things, and is related to them as their prototype or exemplar. For Aristotle, therefore, philosophic method implies the ascent from the study of particular phenomena to the knowledge of essences, while for Plato philosophic method means the descent from a knowledge of universal ideas to a contemplation of particular imitations of those ideas. In a certain sense, Aristotle's method is both inductive and deductive, while Plato's is essentially deductive.

In Aristotle's terminology, the term natural philosophy corresponds to the phenomena of the natural world, which include: motion, light and the laws of physics. Many centuries later these subjects would later become the basis of modern science, as studied through the scientific method. The term philosophy is distinct from metaphysics, which is what moderns term philosophy.

In the larger sense of the word, he makes philosophy coextensive with reasoning, which he also called "science". Note, however, that his use of the term science carries a different meaning than that which is covered by the scientific method. "All science (dianoia) is either practical, poetical or theoretical." By practical science he understands ethics and politics; by poetical, he means the study of poetry and the other fine arts; while by theoretical philosophy he means physics, mathematics, and metaphysics.

The last, philosophy in the stricter sense, he defines as "the knowledge of immaterial being," and calls it "first philosophy", "the theologic science" or of "being in the highest degree of abstraction." If logic, or, as Aristotle calls it, Analytic, be regarded as a study preliminary to philosophy, we have as divisions of Aristotelian philosophy:

• Logic
• Theoretical Philosophy, including Metaphysics, Physics, Mathematics
• Practical Philosophy
• Poetical Philosophy.

Aristotle's logic:
Main article: Aristotelian logic