Existentialist philosophers insist on the ability - indeed, the responsibility - for human beings to create themselves. This is the import of Sartre's famous formulation "existence precedes essence": there is no human essence, and if you believe so then you are engaging in "bad faith" and living inauthentically.
Given all that we have discovered so far about human biology, psychology, and ethology over the last 75 years, these existentialist notions seem rather quaint. Yet even humanistic psychologists who in those days were significantly influenced by the existentialists recognized that Sartre and his ilk had gone too far. Consider this passage from an unpublished manuscript by Abraham Maslow (quoted on page 374 of Scott Barry Kaufman's recent book Transcend: The New Science of Self-Actualization):
Aristotle, Spinoza, Aquinas and others have stressed as a basic law of all being that everything has a tendency and a right to persist in its own nature. The human species has [as] a "species-specific character" the peculiarity that "persisting in its own nature" is a long slow process of growing into one's own nature. It takes a lifetime to become fully-human, i.e., it takes a lifetime for a baby to persist and [become] its own nature ... essence is potential and therefore has to be actualized, made.
Yes, there is much that we can create about our lives, but there are limits. As Aristotle would say, we are social beings, linguistic beings, thinking beings, enculturated beings. Two quotes from Aristotle's Politics are illustrative. First: "He who is unable to live in society, or who has no need because he is sufficient for himself, must be either a beast or a god." (1253a27–29) Second: "What each thing is when fully developed we call its nature." (1252b32-33) Plus, of course, all of our concepts related to "actualization" of "potentials" - or, as I prefer to phrase it, "activation" of "capacities" - comes straight out of Aristotle's metaphysics and biology. This will be a major theme of my forthcoming book Complete Thyself: Aristotle on Human Fulfillment.
(Cross-posted at philosopher.coach.)
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