The way to make understandable how we think, how we see the reality, what we do with this. As for me this is the best example from Ayn Rand:

«…the concept “the science of physics” is not the same thing as the physical phenomena which are the content of the science. The phenomena exist independent of man’s knowledge; the science is an organized body of knowledge about these phenomena, acquired by and communicable to a human consciousness. The phenomena would continue to exist, even if no human consciousness remained in existence; the science would not».