Aristotle categorizes causes into four different types, based on the way in which they necessitate the outcomes that they drive. Efficient causation deals with kinetics and intersubstantial exchanges of energy. Material causation deals with the changes that occur within objects based on their composition. Formal causation deals with the properties of objects based on their forms. Lastly, final causation deals with the actions of objects based on their function or purpose.

Efficient causation, the most straightforward type of cause-and-effect relationship, is based on the simple principles found in Newtonian mechanics. Certain physical properties of substances govern the interaction of large masses, and the way that they work is so intuitive that it barely needs an explanation. When one thing strikes another, the object being struck moves in a predictable manner. Things fall from trees because something always makes objects go down.

Material causation involves the external change of a physical object as the result of its chemical or molecular composition. It is the reason, for Aristotle, why tin rusts, copper turns green, and milk goes sour. Again, little explanation for why it happens, simply admitting as fact that the chemical makeup of an object, which Aristotle considered a form, makes certain things act a certain way. By necessity, when certain conditions are met.

Formal causation covers everything that doesn't fall into one of the more generalized types of causation. Rather than indirectly saying that the form of objects on a certain level of magnitude follows rules dictated by their form (gravity, electro-magnetic attraction/repulsion, friction, etc.), formal causation simply states that individual things behave in certain ways because they have a form that dictates their behavior. Why does Shakespeare's use of words in this sonnet serve as a metaphor for happiness? Because that is one of the forms of that particular word choice. Why does pi represent the ratio of a circle's circumference to its diameter? Because that is one of the forms of pi. Basically a re-statement of the definition of a form. If you are unfamiliar with Aristotle's brand of forms, you should read Aristotle's Doctrine of Substances.

Final causation decrees that certain things act according to the guidelines that lead them to their final purpose, or function. Why does a clock tell time? Because it was built exclusively to keep time. Why does hydrogen have such a small mass? Because it's designed to allow for hydrogen bonding, and indirectly, life on Earth. Why do we eat, and enjoy eating? Because eating makes us live. Just as with the rest of the causations, it is necessary, because in its definition it covers all grounds. It cannot be refuted.

Some people use the Divine Watchmaker theory as evidence for the existence of an active, deliberate God. Aristotle's theory of final causation is reminiscent of this kind of argument, but Aristotle did not believe in the kind of deity recognized by the Judeo-Christian groups today. It could be argued that the presence of a rational animal in the design of the clock suggests a similar rational being behind the design of the human body, or the universe. However, many scientific theories today suggest that certain design plans are formed in nature, without the planning or manipulation of a rational being, and Aristotle would likely side with them.

Since each of the causes according to Aristotle incites necessary reactions in the types of objects it rules over, and since the causes cover all groupings of objects and interactions, Aristotle would be hard pressed to say that any events ever happened by chance alone. The only thing that Aristotle cannot explain is the reason why everything in the world is in a given state at a given time. The events leading up to each state are all accounted for in his causations, but since there is a cause and prior arrangement behind each state of the world, there can never be any initial conditions. Aristotle summarizes this by stating that the universe is eternal. He accounts for the path that the universe has traveled by referring to an unmoved mover, a being whose presence alone guides the world along.