Nicomachean Ethics book 3 chapter 2 concerns choice. Choice is something willing, but not everything done willingly is done by choice. Things spontaneously done by children and animals and things done on the spur of the moment are done willingly and so are subject to praise or blame, but they are not done by choice.
Choice is not desire or spiritedness or wishing or opinion. It is involved with reason and thinking things through. It is the outcome of deliberation, the subject of chapter 3. It is the deliberate desire of things that are up to us (Sachs translation, p.43). It comes from desire combined with a rational understanding that is for the sake of something (p.103); it is “either intellect fused with desire, or desire fused with thinking, and such a source is a human being” (p.104).
We deliberate about things that are up to us and are matters of action. Deliberation is neither knowledge nor opinion. Inquiry about exact sciences or general truths or ends is not deliberation, but deliberation is a kind of inquiry. Deliberation applies to means for achieving ends, when outcomes can be predicted with some confidence, but are still uncertain. On big issues, we consult others. When there is more than one means to an end, deliberation seeks the one that is easier and more beautiful.
Deliberation may also examine how a thing will come about through a particular means, what other means are required for that means, and so on. Aristotle says the analysis of dependencies of means and ends in particular works just like a mathematician’s analysis of a geometrical diagram.
Deliberating well overall belongs to people with good practical judgment (p.112). “What is deliberated and what is chosen are the same thing, except that the thing chosen is already determined, since the thing chosen is what is decided out of the deliberation.” (p.43.) Aristotelian choice is therefore anything but arbitrary. It is a normative rational determination, albeit emergent from an open, fallible, and pluralistic process.