Aristotle gave us a means to perceive this richness of being when, in the process of inventing the science of Physics, he articulated the four causes of all changeable or natural being. To understand—to have the science of—something, you have to know what it’s made of (the material cause), what brought it into being (the efficient cause), what it is essentially or intelligibly (the formal cause), and—governing all of these—what purpose it serves (the final cause). Contrary to the modern philosophers, we do not understand something when we know its matter—that human beings are flesh and bone, for instance. For if bone is our matter, bone is also a kind of form whose material cause is a number of proteins, and those proteins are forms whose materials are a number of elements—and so on. All these material and formal causes hold together only in their composite realization of some purpose: they exist for something; they have a final cause. Saint Thomas Aquinas extended Aristotle’s theory of the four causes to show us that they help us to understand not only changeable being, but the eternal being of God himself.