It depends hugely on the context in which the word appears. Subtle as a synonym of sharp makes just sense in a phrase referring to somebody’s mental strength, making, for example, “subtle disinctions” (and having thus a subtle or sharp mind). But in the context of an effect it can mean “soft”: the measure had just subtle consequences. Yet this does not mean that the two meanings are contradictory. On the contrary, in both cases subtle denotes something minute, but context provides the additional meaning.
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