A philosopher argued 'a white horse is not a horse' — and meant it logically
In the White Horse Dialogue, the Chinese logician Gongsun Long defends the claim that a white horse is not a horse by exploiting two different senses of 'is': belonging to a class versus being identical to it. 'Horse' names a class that includes non-white horses, so 'white horse' isn't identical to 'horse' even though every white horse is one. Legend claims he once talked his way past a city gate that banned horses by riding in on exactly this argument.