What are the premises he’s stating:
…determinism cannot be reconciled with [the fact that] it is sometimes up to me what I am going to do. For if it is ever really up to me whether to do this thing or that, then…each alternative course of action must be such that I can do it….in the sense that it is then and there within my power to do it. But this is never so, if determinism is true, for on the very formulation of that theory whatever happens at any time is the only thing that can then happen, given all that precedes it. It is simply a logical consequence of this that whatever I do at any time is the only thing I can then do, given the conditions that precede my doing it.