In the ethical texts, Aristotle claims that all instances of choice (prohairesis) must be preceded by deliberation, but it is not clear why he believes this. This paper offers an explanation of that commitment, drawing heavily from the De Anima and showing that the account emerging from there complements that of the ethical texts. The view is that the deliberative faculty has the capacity to manipulate reasons combinatorially, while the perceptual/desiderative faculty does not, and choice requires the combinatorial manipulation of (...) reasons. (shrink)