The quality that accompanies the emergence of learning in the evolution of higher animals, namely… — Richard E. Leakey

In its extreme form self-awareness manifests itself in notions such as that of the soul, but in simple form it merely means to be aware of oneself as an individual among others.

A vital leap in the evolution of intellectual capacity would have been the ability to form concepts, to conceive of individual objects as belonging to distinct classes, and thus do away with the almost intolerable burden of relating one experience to another. Concepts, moreover, can be manipulated and this is the root of abstract thought and of invention. The formation of concepts is also a necessary, but apparently not sufficient, condition for the emergence of language.

Ever since Darwin tied knots between human beings and the rest of the animal world, many people have frantically attempted to untie them again, declaring that even though our roots are in the animal world we have left them so far behind as to make any comparisons utterly meaningless. To some extent this is true, because the quality that makes us unique in the biological kingdom is the enormous capacity to learn.