One of the fundamental ideas of Aristotle’s philosophy is that we can understand nature in terms of a series of causes. This in turn is grounded in the idea that nature is ordered teleologically, or ordered to a specific purpose or goal (from the Greek word telos, meaning end or purpose). Aristotle states that “nature does nothing in vain” as a way of explaining why various animals have the particular faculties and senses that they do (212). For example, one will not find an animal that has a mouth but no stomach (250). Thus, to explain a phenomenon in nature, one must find its goal, that “for the sake of which” it exists, which Aristotle terms the final cause of a thing.

This idea is one of Aristotle’s basic assumptions throughout On the Soul. For example, nature supplies animals with the senses they need most for their particular mode of life. Those that are stationary (i.e., plants) have just the nutritive soul, those that move have touch and/or perception, and so forth. Further, each sense organ is intended by nature for a specific function, and from this it follows that sense perception is “never or very seldom deceived” (Introduction, 79) in perceiving special sense objects.