An excerpt from chapter 6 on Man from Objectivism: The Philosophy of Ayn Rand by Leonard Peikoff.
If a fundamental difference is one which has enormous, pervasive manifestations, then the most fundamental difference among the entities we perceive is that between the animate and the inanimate. The starting point in the present inquiry, therefore, is the fact that man is a certain kind of living organism. What is an organism? More specifically, what is its essential, distinctive mode of action?
Read the rest in Objectivism: The Philosophy of Ayn Rand.