What is the Principle of things? What are the building blocks of the universe? Can man know how everything began, and if so, can he perhaps significantly influence their evolution? These are some of the questions that preoccupied the ancient Greek philosophers.

Some of them spoke of the 4 building blocks of Existence (Fire, Earth, Water, Air). Aristotle added a 5th, the Ether, which he exalted to the position of the Quintessence of the universe, defining it indeed as the very material of celestial bodies.

The Ether is different from the other elements of nature as it possesses, according to Aristotle, those properties of an unchangeable, divine, and eternal substance. The Ether, or the fifth element, is that which "inhabits" where the gods are; it is not subject to the process of generation or decay, and it is characterized by the absence of gravity.

It thus moves endlessly and ceaselessly in the realm of the superlunar world, causing the circular motion in celestial bodies. This book presents the entirety of the great Stagirite's views on the quintessence of nature, as well as the criticism he received from philosophers of different schools of ancient Greece.