The variation I personally use is “rationality is the study of how to make good choices.” I think “study of” is doing important work. I usually start with that, and then immediately go on to say “there are two important elements of making good choices—you have to understand how the world works so you know which actions will have which effects, and you have to have the capability of actually taking those actions.”
The variation I personally use is “rationality is the study of how to make good choices.” I think “study of” is doing important work. I usually start with that, and then immediately go on to say “there are two important elements of making good choices—you have to understand how the world works so you know which actions will have which effects, and you have to have the capability of actually taking those actions.”