Causes are cited as answers to questions of the form: "Why is S P?", e.g. "Why is Socrates musical?". The Greek word Aristotle used was aition, for which there is no suitable translation in English; a cause, to Aristotle, was more than just a reason behind an event, it was the reason behind almost any "why" question. A complete answer, to accord with Aristotle's thoughts on knowledge, must take the form:
1) S is M
There are four distinct kinds of cause:
1. Material cause: "that out of which a thing comes to be, and which persists"
2. Formal cause: "the essence," "the account of what-it-is- to-be, and the parts of the account."
3. Efficient cause: "the primary source of change"
4. Final cause: "the end (telos), that for the sake of which a thing is done"
Material and formal causes are preconditions for change, in that they allow for the distinction between matter and form in terms of change. They are static, in that they tell us what the world is like at the moment.
Efficient and final causes explain why things actually come to be what they are. They are dynamic, in that they explain why matter has come to be formed in the way that it has, and in doing so explain change.
Final causes require further elaboration:
1) The final cause of something is its proper functioning, its essence
The essence of something could also be stated as a formal cause (a particular configuration of DNA) or even as an efficient cause (explanation of their DNA and environment as formative in their character). The final cause might today be considered to be 'ensuring its DNA persists', but Aristotle would definitely have said 'to perform its proper function in its community'.
Objection: suggesting natural things, as opposed to man-made things, can have final causes surely places an anthrocentrist or similarly deterministic view on the world. Even if we were to say the final cause of a tree is to absorb CO2 and give out oxygen, we would be implying the sustainability of the earth's ecology determines the worth of every object on that earth. Whilst this might be acceptable to ecologists, it surely raises a serious problem for those who wouldn't subscribe to this deterministic view, nor any other.
This determinism seems to be particularly problematic in nature, but Aristotle attempts to address this. He says that the final cause of an object in nature isn't its purpose or intention, but rather the end of a regular development, e.g. the final cause of a developing tiger is to be a tiger. Natural objects that cannot have final causes in the way that Aristotle claims humans can therefore have final causes that can be identified with formal causes, though that is not to say that at any point in time a natural object's final and formal causes must be the same, since the formal doesn't take into account development and only describes the form at the time, whilst the final would describe the formal at the point of completion of the object's development.
Objection: suggesting everything has a final cause implies that substances have causes posterior to their effects. This isn't necessarily a problem for Aristotle, but it does make final causes seem even more deterministic. One could allow for final causes changing, e.g. a house becomes a tourist attraction, and so its final cause would change from being to be roofed to being to be a good tourist attraction.
Objection: the familiar theme of quantum physics again rears its head, but so too does randomness and chaos theory. Aristotle's theory of change doesn't allow for any randomness nor chance, which certainly coheres with the basis of the scientific methodology, but which runs aground in contemporary physics.