I've heard that Aristotle supposed in his Topics that there's no exact/clear way to distinguish two concepts. What exactly did he say?
Aristotle discusses how to note differences between terms in Topics book 1 §16 (108a1-5):
The differences which things present to each other should be examined within the same genera, e. g. ‘Wherein does justice differ from courage, and wisdom from temperance?’—for all these belong to the same genus*; and also from one genus to another, provided they be not very much too far apart, e. g. ‘Wherein does sensation differ from knowledge?’: for in the case of genera that are very far apart, the differences are entirely obvious.
*of virtue, in this example
Another example: Apples and oranges should be distinguished within the same genus of fruit.
If we consider the genera of non-citrus (of which apples are a species) and citrus (of which oranges are a species), it's obvious that apples and oranges are different.