Practical reasons are reasons for action, while theoretical reasons are reasons for belief. The former direct us to what is good to do, while the latter direct us to what is true about the world. To make this standard distinction is not yet to say anything about the nature of either practical or theoretical reasons, or about how they do or do not relate to one another. In PL Rawls explains that his theory of justice is grounded in a distinctive view about the nature and independence of practical reason, one which Rawls explicitly associates with Kant, and which Rawls calls constructivist.

Rawls contrasts this view with a kind of moral realism illustrated by rational intuitionism (PL 91–94; see also CP 343–346). On this opposing view, practical reasons, or at least moral reasons, are just a special case of theoretical reasons: the moral good is an object of knowledge, “gained in part by a kind of perception or intuition, as well as organized by first principles found acceptable on due reflection” (PL 92). By contrast, Rawls associates his own political constructivism with the (Kantian) view that there are distinctly practical reasons which are not grounded in theoretical knowledge.