If someone were to maintain that all non-mammals are either birds or fish, we could refute him by producing, say, a lizard, and showing that it was not a mammal, but was not a bird or a fish either. If, however, instead of a lizard we were to produce or cite the example of a lump of iron, or the number 7, and say ‘Here is something that is not a mammal, but is not a bird or a fish either’, he would be likely to complain that such an example was beside the point: he had intended his original remark to apply only within the field of living creatures.