I agree that some philosophical searches for analyses of concepts turn out generating endless, fruitless, sequences of counterexamples and new definitions. However, it is not the case that, always, when we are trying to find out the truth conditions for something, we are engaged in such kind of unproductive thinking. As long as we care about what it is for something to be evidence for something else (we may care about this because we want to understand what gives support to scientific theories, etc), it seems legitimate for us to look for satisfactory truth conditions for 'e is evidence that h'. Trying to make the boundaries of our concepts clear is also part of the project of optimizing our rationality.