How many books can claim to be so influential as to inspire the development of a whole school of thought? The ancient Greek philosopher Aristotle’s Metaphysics did exactly that, laying the foundations for a new branch of philosophy—metaphysics—concerned with the cause and nature of being.

Aristotle questioned his teacher Plato’s renowned Theory of Forms. Plato argued that everything in the world is nothing more than an imperfect representation of its perfect form. Aristotle, however, focused on the reality of the material world. He argued that anything that can be asserted about anything that exists is due to its actual existence. In other words, its substance is in its present form. But Aristotle went further. In analyzing this substance, he then introduced more philosophical ideas—like the “unmoved mover,” potentiality, and actuality—that also became hugely important in their own right.

Ancient philosophers and contemporary thinkers have all cited Aristotle’s ideas as an inspiration, which is why Metaphysics is still such an important work today.