If the reasons are external, and preprogrammed, it would be incorrect to call them "one's own" reasons. I think that the "reasons" in the form in which they are attributed to the individual, would be distinctly different from the "reasons" which were prior to the individual, so we could not say that these are "the same reasons" in a different form. They would be distinctly different reasons. And if they are by any means "the same reasons", then we cannot attribute them to the individual.
It seems to me that in many ancient and medieval ethical systems it would be both. There is on the one hand man's telos, which is internal to man (plural), but determined prior to any individual man. On the other hand, there is free man's own reasons for doing what he does, being who he is etc. The whole reason ethics is difficult is that these two
can vary from one another in practice. Man can fail to fulfill his telos and fail to flourish, through his own
choices.
The ideal situation is where man's free choice synchs up to mankind's telos. But there is a wrinkle here in that these thinkers were generally not free will libertarians. However, neither were they modern fatalists. Rather, they embrace a certain sort of "classical fatalism." "Character is destiny," Heraclitus says. They embrace the concepts of "fate" and "divine providence," and elucidate the ways in which man is a slave to circumstance, desire, and instinct, and yet allow that man, both individually and as a society, can manage to become more or less free / self-determining. Part of fulfilling man's telos
is precisely becoming more self-determining and more "one's self," rather than being a mere effect of external causes. (Modern existentialism recapitulates part of this, while missing crucial elements)
This is why an overflowing love is important in Plato and the Patristics. To hate something to be controlled by it. To be indifferent to something is still to be defined by what one is not. Only love, the identification of the self in the other, allows one to avoid being determined by what is external to personal identity. This translates into a "love of fate," that must accompany the entity that will not be mere effect.
Plato's early-mid works are instructive here. The Phaedo and Republic goes into how man becomes more self-determining by overcoming desire, instinct, and circumstance, a view we also see particularly well elucidated by Saint Paul in Romans 7. For both Paul and Plato, Logos plays an essential role in the resurrection of the "slave to sin," to true personhood. However, in Plato there is also more of a recognition of the ways in which we enable each other's freedom, and in how society can enhance or fail to enhance to fulfillment of man's telos (the Apology and Crito get at this social dimension).
I think the social view moves towards a climax in Eusebius, who has a proto-Hegelian view of how history can act as an engine spurring man on towards the greater fulfillment of human purpose at the world-historical scale. With the medievals, you also start to see the acknowledgement that, while human telos has certain unchanging elements, it is also shaped by the social-historical conditions man finds himself in.
So individual man's reasons are not identical with the the global telos of man. This is precisely because man is not free, and being enslaved to desire, ignorance, and circumstance , man lacks the knowledge and means of fulfilling his purpose. Even modern existentialists seem to recognize the need for some level of self-determination to make life meaningful, although they deny the global telos.
The shift to emotivism is important here. For the existenialist, moral freedom can't be the crowing achievement of man because moral freedom is simply reducible to desire. Due to their focus on the individual, they often lack the same focus on social freedom as well, but not always. Without these, the idea of a telos for mankind does indeed become incoherent and reduce to a single "internal" purpose defined only by the the individual.
Interesting that he converted to Catholicism from having been a convinced Marxist.
Interesting, but not totally surprising. There is this whole huge area of Catholic philosophy that sits sort of free floating from the rest of academic philosophy. It tends to be far more focused on ancient/ medieval philosophy, but unlike secular academic philosophy re ancient/medieval philosophy, it is also intent on updating these for modern times.
This camp does produce a lot of good philosophy. E.g., Nathan Lyons "Signs in the Dust," is the best theory of pansemiosis I've found, and is far more grounded in the natural sciences and much less "heavily continental," than anything else I've seen attempt this sort of thing. Sokolowski's "Phenomenology of the Human Person," a blend of Aristotle and Husserl, that also takes the natural sciences and modern linguistics seriously is another example. It's one of the better articulations of a "(more) direct realism."
Unfortunately, the conversation between these camps seems to mostly go one way ("After Virtue" being an exception), without much back and forth.