...A young child has no reason to think of horses, fish, and chickens as examples of the same general category. But if we tell the child, "An animal is, for example, a horse or a fish or a chicken," it's essential that the child be able to summon horse, fish and chicken memories, overlay them, and consider the common features that emerge. Those common features may be hard or impossible to capture in words, but whatever they are, they form the basis of the child's understanding of the concept animal.
A concept like blue makes an even clearer instance... There is no way...to define blue without recourse to memory overlays. Blue simply is whatever common feature emerges when many blue memories are overlaid.