One of the most famous statements of humanism from classical antiquity was that of Protagoras, who said, "Man is the measure of all things." This statement places human nature at the center of any rational system or set of scientific principles. Socrates made the subject of virtue the basis of his philosophical inquiry and Aristotle launched the first systematic investigation of moral psychology. This general belief in a fixed human nature contrasts, for example, with the work of Jean-Paul Sartre, who stated that there is no single human nature, but only radical freedom.
Works of humanist art, from the sculpture of ancient Greece to the paintings of the Renaissance, emphasized the geometric perfection of the human form and held up exemplars of beauty as objects to inspire something akin to religious devotion. Existentialism rejects this approach because it sees the human form as deriving from adaptation and circumstance rather than ideal standards and because it subjects claims about beauty to the criterion of creative engagement with the circumstances of life.
The 19th century German humanism of Hegel, Fichte and Marx issued in various formulations about human reason and productivity as the inevitable goal and destiny of history. Hegel saw human beings as passing through stages on their way to actualizing Spirit in history and Fichte searched for signs of a pending climax of historical epochs. According to existentialism, history is an epiphenomena whose purpose has not yet been decided, and human beings cannot be its subject because the meaning of the human is still unknown.
Classical humanism depends on the metaphysical formula of essence preceding existence. According to Martin Heidegger, existentialism reverses this by saying that existence precedes and exceeds essence, but does so without proposing a new metaphysical system. In his existential phenomenology, the analysis of Dasein can take place before deciding any metaphysical questions such as whether there is an objective world with beings in it.