The opening line of the book – "It is possible I already had some presentiment of my future" (p. 9) – signals that some kind of "destiny" or "fate" is at work in Severian's life. His future is set. We are forced to ask who, or what, has set his future.
There are implications of divine providence throughout The Book of the New Sun, with constant reference to the Pancreator / Increate (God / Holy Spirit) being sovereign over all things. However, we are never given any conclusive evidence of this one way or the other.
Later in the book, we find that highly advanced alien creatures (cacogens, Hierodules, giant sea creatures, etc) have been interfering in Severian's life from very early on – perhaps before he was even born. Severian's manipulation by Agia and Agilus throughout The Shadow of the Torturer may serve as a parallel for his manipulation by powerful alien forces throughout the entire story.
The free will / determinism problem is further complicated by the introduction of time travel – especially Severian's ability to step into the "corridors of time" and go into the past, thereby altering the future. At the end of The Citadel of the Autarch, Severian speculates that he was not the first Severian, and that there have been many Severians before him who have gone into the past and changed the future (his life). We find in The Urth of the New Sun that these alterations to Severian and his life were all for the purpose of having him pass the test necessary to bring the New Sun (which previous incarnations of him had failed). Perhaps Severian, through time travel, has taken his own free will away?
Another way in which determinism works its way into the story is through Severian being driven by his own desires – yearnings that he is not fully in control of. Severian discusses this in one of my favourite passages from The Shadow of the Torturer, which comes as he discusses the difficult exegesis of the tale of Ymar:
The difficulty lies in learning that we ourselves encompass forces equally great. We say, "I will," and "I will not," and imagine ourselves (though we obey the orders of some prosaic person every day) our own masters, when the truth is that our masters are sleeping. One wakes within us and we are ridden like beasts, though the rider is but some hitherto unguessed part of ourselves. (p. 159)Severian, it seems, regards his actions as predetermined by his inner desires, which "wake" and "ride" him. These desires appear to be beyond his control. This sounds like a fatalism which sees the human being as entirely determined by their biology and their experiences – nature and nurture.
Whichever of these be the case, we are given ample evidence of the determinism at work in the narrative. With implications that events have been set by biological and cultural determinism, time travel paradoxes, manipulation by more powerful beings, or divine providence, Severian never seems to be fully in control of his actions, or the events in which he is involved. But then again, who is?
Quotations are taken from: Gene Wolfe, The Shadow of the Torturer, in The Book of the New Sun – Volume 1: Shadow and Claw, Fantasy Masterworks edition (London: Gollancz, 2000).