He does this by using four causes, or conditions, for a complete explanation.
The ‘Telos’ cause; (that for the sake of which, purpose, end, completion)
The ‘Formal’ cause; (what it is, whole, organization of parts)
The ‘Whence’ cause; (from which, generation, source, initiation) once called ‘efficient’
The ‘Material’ cause; (out of which, matter, stuff, parts)
The difference between living and non-living things is the fact that the location of the source of motion is intrinsic to the ‘living thing’ itself. When it comes to humans, where a person is ‘coming from’ also makes a difference, since the ways we think is ‘acquired’ by living (second nature).
This ‘second nature’ is the habits we have towards pain and pleasure, and we can be ‘deficient’, ‘excessive’ or ‘properly’ habituated to various circumstances. We can potentially learn to balance between being a habitual ‘boor’ (habits of deficiently pursuing pleasure), or a ‘fool’ (habits of excessively pursuing pleasure), as well as a balance between being too ‘hard’ on oneself (habits of deficiently avoiding pain) or by being too ‘soft’ on oneself (habits of excessively avoiding pain).
The mark of a habit is that for the sake of which a habit aims. The confusion comes from what the individual understands the ‘mark’ to be, and where he stands in life based on this understanding, which is ‘where he is coming from.’ Since facts don’t come labeled as such, how we proceed to organize our experience takes an understanding that seems to include ‘purpose’ that doesn’t exist in today’s sciences. It’s our point of view.
The atomists only used the material cause, so it all reduces down to the absolute ‘least part.’ The Sophists only use the ‘whence’ cause, so they proceed by backtracking to arbitrary beginnings. The Platonist’s use the ‘formal’ cause, which rises to the formal identification of something (dead or alive). The term ‘psyche’ is translated ‘soul’, but it seems closer to life; that difference between ‘meat’ and a ‘living’ animal.
The ‘life,’ the psyche/soul, of the organism is that for the sake of which it exists (not merely it’s matter, whence it came, or form – which can all exist with dead things). For the end of life is not its purpose.
Rather, each of the other three causes are incomplete for fully explaining a ‘living thing’ without the cause ‘for the sake of which’. Why does it rain? Not for the sake of anything. Rain is just heavier than the air – so it falls to earth. In this sense it only has a material existence. But since rain is one of the material conditions necessary for a form of matter to be alive, life won’t exist without it. Water is one of the material causes (preconditions) for life.
But the exactness that comes with a mere material explanation is partially lost when explaining human actions with a ‘that for the sake’ type of explanation. Otherwise, Aristotle says, all is in vain. Which I suppose might be the case. But his understanding is the only game in town that includes personal responsibility.