I had supposed that predetermination was the opposite of free will.
Ah, that's determinism. Determinism is something we could never know the truth of any more than free will, unless it is understood that we are the ones who determine the order or sequence of things. We experience here and now; we do not experience the past and the future. Here and now, happenings are both causes of future events and the effects of past events; but their effect on the future is only determined by probability, and their cause is assigned, by us, in the same manner. Cause and effect is a 'string' of probability that we assemble, thread by thread, and weave in consciousness (in "knowing").
> So if all our decisions are based on our perceptions at the time, and "free will is not the perception of free will", how do we decide that "free will is not the perception of free will"? <
Our perceptions happen here and now. Our free will happens here and now. If, here and now, we perceive a reality to the freedom of "I own this body," or "I own this thought," "they are mine and I take responsibility for the things done with them," then that is the aspect of free will I was looking at. It does peripherally involve a determined world, yes (hence, while "opposites", they cannot be mutually exclusive).