to understand what a human being is it is not enough to 'know'...

 To understand what a human being is what is require is an anti-system sensibility, its flight from the “iron cage” of reason

for it is not enough to know all the truths that natural science—including the science of psychology—could tell us. 

The dualist who holds that human beings are composed of independent substances—“mind” and “body”—is no better off in this regard than is the physicalist who holds that human existence can be adequately explained in terms of the fundamental physical constituents of the universe.

 Existentialism does not deny the validity of the basic categories of physics, biology, psychology, and the other sciences (categories such as matter, causality, force, function, organism, development, motivation, and so on). It claims only that human beings cannot be fully understood in terms of them. 

Nor can such an understanding be gained by supplementing our scientific picture with a moral one. Categories of moral theory such as intention, blame, responsibility, character, duty, virtue, and the like do capture important aspects of the human condition, but neither moral thinking (governed by the norms of the good and the right) nor scientific thinking (governed by the norm of truth) suffices.