But how is it that, when we think of something, sometimes we take action and sometimes not, sometimes we are in motion, and sometimes not? It seems to be nearly the same case as when we consider and think of things that are unchangeable (as mathematics is). But in that case the end is the speculation deduced (for in reasoning as soon as we consider the two premisses what we conceive and infer is the conclusion), whereas here the conclusion deduced from the two premises is the action, as for example, when we think that every man should walk, and that we ourselves are men, straightway we walk, or again if we think that no one should walk now, straightway we cease; and we do both of these, unless something prevents or compels us. I have to make something good; a house is something good. Straightway we make a house. I need a covering; a coat is a covering. I need a coat. What I need I have to make; I need a coat. I have to make a coat. And the conclusion, that I have to make a coat, is an action. And the end of the reasoning is the point whereat the action begins. If it be a coat, it must be first this, and if this, then that; and straightway we do that. That indeed the action is the conclusion is obvious; while the premisses conducting to action are of two kinds: we consider whether this is good or whether it is possible.
And just as in a dialectical argument some do not ask the obvious premise, so also here the mind does not pay at all any attention to consider the other premise the obvious one. For example, if walking is good for man, it does not waste time in examining if he is a man. And this is why whatever we act without thinking we do it quickly. For every time we act toward a cause whether by perception or by imagination or mind, we straightway do what we desire; for instead of questioning ourselves or thinking about we act according to desire. “I have to drink”, says desire. “Here is a drink”, says perception or imagination or mind. At once we drink. In this way, then, living creatures impel themselves toward some motion and action, being desire the ultimate cause of motion, since it comes about either by means of perception or by imagination and thinking. And of things we wish for, some we do, others we act, some by desire or impulse and others by appetite or wish.
Bibliography: Aristotle On movement of animals (701a.7 to 701b.1)
Translation: George Kotsalis