Given determinism, you can say that something is possible, that it can and does happen at some time.
The problem is not with things that "can and do happen". Our issue involves things that "can and don't happen". Are things that "can" happen, but "will not" happen, to be considered real possibilities or not? That's what we're focusing on here.
In order for us to process matters of uncertainty in a rational manner, we require some logical token that refers to things that inevitably will not happen as well as the things that inevitably will happen. The notion of a "possibility" serves this purpose.
Something is possible if we are able to do it. It is not necessary for us to actually do it in order for it to be a real possibility.
But the point being, given determinism, when it happens, it happens necessarily. Whenever the 'possible' happens, it must happen.
Of course. But, again, what are we going to call the things that
could happen but necessarily
will not happen? My point is that we have already solved this problem. Possibilities remain possibilities whether they are determined to happen or determined not to happen.
When the 'possible' happens, it must happen as determined, and nothing can happen in its stead.
Nothing else
will happen in its stead. However, other things could have happened in its stead. I chose the Salad, but I
could have chosen the Steak instead.
This is why the Britannica description is incorrect, and other descriptions of determinism that claim that "we
could not have done otherwise" are all incorrect.
I could have chosen the Steak is a
fact. It is a fact because "could have" always implies (1) that it
did not happen, which is a fact because I did not choose the Steak. And (2) that it
only would have happened under different circumstance, which is also a fact, because it only would have happened if it had been determined that I would order the Steak rather than the Salad.
The notion of multiple possibilities is essential to human creativity and invention as well as to choosing. And it continues to do its work in a world of perfectly reliable cause and effect, by shifting to a different language and logic specifically evolved to deal with such matters.
Multiple possibilities exist in the sense that these things can and do happen within the system.
And many possibilities also exist in the sense that these things can happen but never will happen within the system.
That is the point. Multiple things "can" happen, even though only one thing inevitably "will" happen and the rest inevitably "will not" happen.
That fact that something will not happen does not mean that it is impossible.
... Space travel is possible, for instance, but was never possibly before the necessary technology was developed. It was not possible for the Greeks, Romans, etc, but it is for us because that is how history and it events unfolded.
Good. The space travel example demonstrates what "impossible" means. If something is impossible at a given place and time, then it cannot happen. As you point out, space travel was impossible for the ancient Greeks and Romans. They could not fly to the moon even if they chose to. So, no one, except the mentally ill, chose to do so. It was not in their list of realizable alternatives. It was not on anyone's menu of options.
Something is "possible" if we are able to do it if we choose to. Something is "impossible" if we are unable to do it, even if we chose to.
While I cannot fly to the moon, I can walk to the kitchen. Walking to the kitchen is a real possibility, and it
remains a real possibility even if it is deterministically inevitable that I will not choose to do so right now.
I know it is a real possibility, because earlier I walked to the kitchen and fixed a cup of coffee. It is always something that I can do, anytime that I choose to. Determinism does not change this.
That is in accordance with your own definition.
All of this, that I am saying right now, about possibilities and about what I could have done, is in accordance with my definition of determinism, and in accordance with the first sentence in the Britannica definition, and in accordance the the Tech definition (same inputs, same outputs).
I chose the Salad, but I could have chosen the Steak. Both parts are factual, and are also entirely consistent with the definition of determinism that I am using.