First and foremost, a presupposition must be clarified and then accepted, namely, the belief that a man "ought to". In other words, not everything in his action and behavior is well and good just as it is. It makes no sense trying to convince a pig it ought to act and behave "like a real pig".
That the rude line by Gottfried Benn -- "The crown of creation: the pig man" -- can be spoken at all and, further, hold true in such terrible ways: this fact alone shows that humanity must still realize the truly human in the domain of lived realities; it means man, as long as he exists, "ought to".
Of course, one can formulate the concept somewhat less agressively than Gottfried Benn. In this way, for example: "Fire does by necessity what is true and right according to its being, not so man, when he is doing the good." This is a sentence from Anselm of Canterbury's DIALOGUE ON TRUTH. Two statements are there by made about man. Man, on the one hand, is free; and, on the other hand, meaning is given to him regardless of his opinion or his permission.
It is precisely this last fact that all existentialism resists and, as it reaches far beyond the domain of a special philosophical school, also determines the common attitude of the people of our time; this is exactly what Jean-Paul Sartre's famous sentence means: "There is no such thing as human nature!"
To one who does not acknowledge that the human bieng "is" homo sapiens in a totally different manner than water "is" water; that, to the contrary, the human being ought to become what he is (and therefore not already, eo ipso "is"); that one can speak of all other earthly creatures in the indicative, in simple statements, but of man, if one wants to express his actual reality, one can only speak in the imperative -- to him who cannot see this or does not want to admit to its truth it would be understandably meaningless to speak at all of an "ought to" and it would make no sense to give instructions or obligations, be it in the form of a teaching on virtue or otherwise.(2)1
(2)1 Originally published in "Die Aktualitat der Kardinaltugenden", BUCHSTABIER-UBUNGEN (Munich: Kosel-Verlag, 1980). Translated by Margareta Svjagintsev.