Erich Fromm

What is it that distinguishes man from animals? It is not his upright posture. That was present in the apes long before the brain began to develop. Nor is it the use of tools. It is something altogether new, a previously unknown quality: self-awareness. Animals, too, have awareness. They are aware of objects; they know this is one thing and that another. But when the human being as such was born he had a new and different consciousness, a consciousness of himself; he knew that he existed and that he was something different, something apart from nature, apart from other people, too. He experienced himself. He was aware that he thought and felt. As far as we know, there is nothing analogous to this anywhere in the animal kingdom. That is the specific quality that makes human beings human.