Practical reason
Reasoning about what to do, as distinct from reasoning about what is the case.
What it commits you to
Practical reason is the faculty that determines ends and evaluates means, and Kant's argument is that it reaches the unconditional through a route distinct from theoretical knowledge. This is the framework in which a moral law can bind an agent that knows perfectly well what is going to happen to it. Systems that optimise do not have it, which is either a design choice or a category mistake.
Which realms lean on this
Philosophy carries the arguments the other four kinds are downstream of — what an agent is, what a person owes, what a rule is for.
philosophy · 8 realms — The Care Weft, The Contractual Court, The Existential Wilds, The Kantian Citadel, The Longview Foundry, The Middle Kingdom, The Pragmatic Wharf, The Sangha Terraces.
Canon that discusses this
primary · 1781
Critique of Pure Reason
The first Critique bounds theoretical reason: the synthetic a priori is real, and it reaches only the phenomenal. The second edition of 1787 reorganises the book around the transcendental deduction. The famous closing passage about the starry heavens and the moral law belongs to the Critique of Practical Reason (1788), not this one. The excerpt below paraphrases the programme rather than quoting a translation.
primary · 340 BCE
Nicomachean Ethics
The first sustained attempt to say what a life is for. Happiness is the end, and it is achieved by habituated virtues exercised with practical wisdom, in the particular and under uncertainty. No manuscript carries an authorial colophon and the dating is reconstructed from cross-references, so 340 BCE is a conventional anchor.
See also
Categorical imperative
philosophyKant's test of an act's maxim by asking whether it could be a universal law.
Hypothetical imperative
philosophyA command that binds only if you happen to want the end it presupposes.
Normativity
philosophyThe property of being able to count as a reason for action.
Pragma
philosophyThe Stoics' term for a maxim of conduct meant to guide action rather than to describe the world.