(I would have said: "When a concept is inherently fuzzy, it is a waste of time to give it a definition with a sharp membership boundary.")
Thus we merely require citizens to "be responsible adults" before they can vote rather than give a sharp boundary such as 18 years old, college applications tell you "don't write a long, rambling essay" rather than enforce a 500-word limit, and food packaging specifies "sometime in September" for the expiration date.
Sharp membership boundaries are useful to make it easy to test for the concept. Even if the concept is fuzzy and the test is imperfect, this doesn't need to be a waste of time.
Though sometimes it's even more useful to acknowledge that the sharp-boundaried concept we're testing for is different from, though perhaps expected to be correlated with in some way, the fuzzy concept we were initially interested in.
That helps us avoid the trap of believing that 17-year-olds aren't responsible adults but 18-year-olds are, or that 550-word essays are long and rambling but 450-word essays aren't, or that food is safe to eat on September 25 but not on September 29. None of that is true, but that's OK; we aren't actually testing for whether voters are responsible adults, essays are long and rambling, or food is expired.
Another month has passed and here is a new rationality quotes thread. The usual rules are: