Causality

Aristotle (384–322 B.C.E.) claimed a sharp distinction between understanding the fact and understanding the reason why (dioti; aitia). Though both types of understanding proceed via deductive syllogism, only the latter is characteristic of science because only the latter is tied to the knowledge of causes. In his Posterior Analytics, Aristotle contrasted the following two instances of deductive syllogism:

1. Planets do not twinkle; what does not twinkle is near; therefore, planets are near.
2. Planets are near; what is near does not twinkle; therefore, planets do not twinkle.

Syllogism A demonstrates the fact that planets are near but does not explain it because it does not state its causes. On the contrary, syllogism B is explanatory because it gives the reason why planets do not twinkle: because they are near. Explanatory syllogisms like B are formally similar to nonexplanatory syllogisms like A. Both are demonstrative arguments of the form: all Fs are Gs; all Gs are Hs; therefore, all Fs are Hs. The difference between them lies in the "middle term" G. In B, but not in A, the middle term states a cause. As Aristotle said: "The middle term is the cause, and in all cases it is the cause that is being sought" (90a5–10). To ask why F is H is to look for a causal link joining F and H. Aristotle's key observation was that, besides being demonstrative, explanatory arguments should also be asymmetric: the asymmetric relation between causes and effects should be reflected in an explanatory asymmetry between the premises and the conclusion of the explanatory arguments—the premises should explain the conclusion and not the other way around.

Aristotle took scientific knowledge to form a tight deductive-axiomatic system whose axioms are first principles, being "true and primary and immediate, and more known than and prior to and causes of the conclusion" (71b20–25). Being an empiricist, he thought that knowledge of causes has experience as its source. But experience on its own cannot lead, through induction, to the first principles: these are universal and necessary and state the ultimate causes. On pain of either circularity or infinite regress, the first principles cannot be demonstrated either. So, something besides experience and demonstration is necessary for the knowledge of first principles. This is a process of abstraction based on intuition, a process that reveals the essences of things—that is, the properties by virtue of which the thing is what it is. In the example B above, it is of the essence of something's being near that it does not twinkle. In the rich Aristotelian ontology, causes are essential properties of their subjects and necessitate their effects. He thought that the logical necessity by which the conclusion follows from the premises of an explanatory argument mirrors the physical necessity by which causes produce their effects.

In his Physics, Aristotle distinguishes between four types of causes. The material cause is "that out of which a thing comes to be"; the formal cause is "the definition of its essence"; the efficient cause is "the primary source of the change or rest"; and the final cause is "that for the sake of which a thing is done" (194b23–195a3). For instance, the material cause of a statue is its material; its formal cause is its form or shape; its efficient cause is its maker; and its final cause is the purpose for which the statue was made. Aristotle thought that a complete causal explanation has to cite all four causes: the efficient cause is the active agent that puts the form on matter for a purpose.