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Sidgwick goes on to argue that the best going account of ultimate goodness is a hedonistic one, and that this is an informative, non-tautological claim, though also a more controversial one than many of the others that he defends.
If the reasoning were valid, we should expect all tautological sentences to have non-tautological implicatures; but such implicatures are the exception rather than the rule.
What’s relevant is that the fittest survive long enough to produce offspring, and thus their fitness descends given that there are non-tautological processes that allow that.