The faculty of reason concerned with action and decision-making, particularly in moral matters. In Kant's philosophy, it's the source of moral principles and the categorical imperative.
From Greek 'praktikos' (concerned with action) and Latin 'ratio' (reason, calculation). Aristotle first distinguished practical from theoretical reason, and Kant made it central to his moral philosophy.
Practical reason is like having a moral GPS built into your brain—it doesn't just figure out what's true, but what you should do. Kant thought this was humanity's superpower, the thing that makes us moral agents rather than just smart animals.
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