Must principles of justice be practical? Some political philosophers, the “implementers,” say yes. Others, the “idealists,” say no. Despite this disagreement, the implementers and idealists agree on what “practical” means, subscribing to the “implementation-prediction” (IP) conception of practicality. They also seem to agree that principles of so-called “ideal theory” need not be (and often are not) IP-practical. The implementers take this as a reason to reject ideal theory as an approach to principles of justice, while the idealists do not. In this paper, I argue that we should reject the IP conception of practicality. The implementers make a mistake, then, by requiring principles of justice to be IP-practical. But the idealists make a mistake, too, by rejecting in general the requirement that principles of justice be practical. For there is a plausible alternative conception of practicality that political philosophers should accept: the “experimentation-learning” (EL) conception. EL-practicality makes for a more realistic and epistemically accessible standard of practicality, and thus should be welcomed by the realistically-inclined implementers. It also preserves a crucial role for ideal theory, so should be welcomed by the idealists, too.
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