Consider two otherwise identical worlds, in one of which determinism is true, and in the other of which it is not. In the second world there is freedom of the will. What difference does this make to the legal system and the moral system? Ex hypothesi, none. When I wrote that if hard determinism is true, moral systems cannot be corrupt, I was describing Stephen's view, not my own! Hence I think the apparent contradiction that he detects. Here's another way of putting the point I wanted to make. We can imagine a deterministic world in which people are thinking good thoughts and doing good things, and making good laws. The world of law and morality seems to have survived the loss of freedom! In this world, some philosophers believe in determinism, and some in freewill. But of course the having of those beliefs is part of a deterministically produced system. There could be incidentally be a deterministic world that is morally and legally better than a world with freewill. Consider the worst world imaginable, but having freewill, and the best world imaginable, but lacking freewill. You might prefer to live in the unpleasant world, but that doesn't mean it's better. In it the innocent are tortured, unfairness abounds, and so on.
The principle of sufficient reason, due to Leibniz, states that there is always a reason why some particular thing happens, rather than some other thing. This does not immediately or obviously pose a threat to freedom. Note that "reason" does not mean the same as "cause", although a cause might be a reason.
Determinism states something much stronger, more complicated, and more sinister. It tells us that the laws of nature and the initial state of the universe at some time in the past entail the state of the universe in the present. Entailment is a strong relation, and what determinism means is that if the laws are whatever they are and the initial state of the universe is whatever it is, then the the universe must (nota been, "must") go into the subsequent state. There is a necessary truth. It is that if the universe is in the initial state, and the laws apply, then the universe will go into the second state. Determinism has been held to pose a severe threat to freedom in the metaphysical sense, or freewill, though so-called compatibilists have a view which de-fangs determinism, if it is successful.
The principle of universal causation is different again. It states that every event has a cause. This might well be false of the first event in time, if it really is the first event, because there is no prior event available to cause it, since nothing is prior to the first event.
To me it seems that the use of the word "can't" and its meaning is the same in all three settings (moral, prudential and physical). "Can't" means there's a contradiction in saying that you do the thing you are said not to be able to do. But the contradiction does not appear without the addition of the "laws" or principles of ethics, or the statement of what one wants, or the laws of physics. There is one exception to this principle. "Can't" in logic and logic-derived fields asserts a contradiction without any body of auxiliary propositions. 'I can't lift 1000kg' means that given the facts of my strength and some facts of physics, there is a contradiction in saying that I lift 1000kg.
You make an important observation for compatibilism. What your analysis does is to show that we can have predictability and law (in a regularity sense) with no implications for individual freedom. My decision to cross the railway track might lead to my death, and it might produce a number that fits the predicted number of deaths on railway lines in a year. Was my decision then not a free one? Hardly, because for that to be the case it has to be coerced or whatever the particular compatibilist line being taken is. The fact that there are h homicides a year in the United States, on average, and that without the homicide I commit the number would be h-1 has no relevance to the freedom or unfreedom of my act. Your point was of central importance to the classical compatibilists, who realized that knowledge and predication of what will happen have no tendency to undermine freedom. I know what I will do, but this could hardly be a reason to say what I will do is not free. It might even be a condition for it to be free!
No, it's a mistake to say that you are choosing not to do all those other things. When you decide to ask someone "What's your name?" it is not "accurate" or right to say that you have chosen not to ask that person "Where are my walnuts?" What is true is that you are not choosing to do the infinite number of things, and you are not choosing to ask where your walnuts are, but that is not at all the same thing as choosing not to ask where they are. Not choosing to ask my name is not a choosing, whereas choosing not to ask my name is. You can also see that it couldn't be right to say that you are choosing not to do infinitely many things - there just isn't time!
We tend to regard the will as something that is marked off from the rest of the person, because, somehow, it is a direct manifestation of the person's being. So an ADHD drug could not be described as "strengthening people's will", because it if were described in this way it could not then be said to be be their own will that was being strengthened; they would be having it down for them.
Similarly, one might think, you can do my work for me, but not my thinking, because then it would not be my thinking that was being done. (Still, in that sense you would not be doing my work - my working - and it is just as impossible for you to do my work as it is for you to think my thoughts or even perhaps to wear my boots, taken to be the ones I am wearing ("Look, his boots (borrowed boots) have mud on them")).
One might on the other hand regard the will as the energy or strength to carry something through. Or one might regard it as determination, though here too the paradox shows through. If my determination is bolstered by a drug, is it really determination? But one can surely lack psychic energy or strength for just the sort of reason (anaemia, say) that one lacks physical energy. In such a case, one could well speak of "strengthening a person's will" or ability to carry through.
It seems to me possible by the way that one could learn from a drug, as Peter Kramer seems to imply can happen with Prozac, in his 1993 book Listening to Prozac. So the possibility seems to exist that the drug can actually teach someone who lacks it what strength of will is, and perhaps then they could do more easily "on their own" later.
Your problem is very interesting and difficult.
I hope that you are wrong in your account of what Stephen Hawking writes in The Grand Design, because it is so obviously wrong and uninformed. There is no freewill, Hawking writes, according to you, and the reason is that stimulation of particular regions of the brain results in certain desires, such as a desire to move one's right arm.
Consider an analogy. One might want to argue there is no such thing as a free or random roulette wheel, because magnetic "stimulation" of some number on the wheel will make the ball want to land there. Of course to say that a human action is free is not to say that it is random, but to say that a human action is free has something in common with saying, of a roulette wheel, that it is not rigged or that the ball is somehow forced to land where it does.
From the fact that I want to raise my arm when my torturer's make me want to does not show that when I am not being tortured my desire to reaise it is not free. Even if some human actions are not free, in cases where there is stimulation of the brain, how does this have even the slightest tendency to show that, in cases where there is no stimulation of the brain, there is no freewill?
And finally, it is vary hard to evaluate what Hawking writes if he does not tell us what "free" means or what it means for a "will" to be "free." Hawking is a determinist, so one can infer that he believes that if something has a cause then it is not free. (This proposition is called the "incompatibility thesis", as it asserts the incompatibility of the predicates "free" and "caused".) However, if Johnny is caused to eat his soup by his hunger, that hardly means that his eating is unfree. For that to be the case, a minimum condition is that he is unable not to eat his soup, i.e. that his hunger is so extreme as to force him to eat his soup, or perhaps that a parent is threatening him with a wooden spoon and making him eat up his nice soup, or else.
Of course Hawking may be right in what he asserts, but the argument in the passage you describe does nothing to show that this is the case.
Here is a side note to your question. Soft determinism consists of two propositions: (1) the the thesis that determinism is true; (2) that it is compatible with freedom. Compatibilism on the other hand is merely (2). So soft determinism includes compatibilism, but there is more to it. I am a compatibilist but not a soft determinist (I am a compatibilist indeterminist), as I believe that there are some events that have no causes (denial of universal causation), and I also believe that the state of the universe plus the laws of nature do not determine the next state of the universe (determinism), and I also believe that some human actions are free. The only other compatibilist indeterminist I know of is David Lewis.
The fact that our actions are predictable as such or by itself hasno tendency to imply that they are unfree or that they are determinedor not freely chosen. I can predict that I will continue to work on myhouse yet again today, but that is because I very much enjoy it, andthe action's predictibility does not somehow frustrate its freeness.Many people are predictably kind, say, but that does not mean thattheir kind actions are not less free and praiseworthy. There may ofcourse be others who are naturally kind, but even they, it seems to me,can well be thought of as free and as deserving praise for their moralquality. It is not the absolute predictability of the motions of theplanets which makes us suspect that they are not free agents, but thefact that we know that they are great balls of rock or magma orsomething. (The philosophical discussion of "compatibilism" over the last hundred years is very helpful here.)
Ithink of culture as a very natural but very human attemptto find or create something good and interesting where it was notbefore. Architecture is an example of a bit of practical culture whichis not just about concealing the long dull littleness of life. It is rather aboutmaking something attractive and humanly satisfying. (I am not sure howidentity comes in here.) But your lastremark does seem to be a bit of what I believe magicians callmis-direction. Is that the right word? Again, predictabiliy is not thesame as dullness. A child's life can be utterly predictable and yetvery exciting. Adults tend to lose their edge. Good domesticarchitecture helps us to remember and restore ourselves. Beaver cultureis not an attempt to escape dullness, but a creation of a pleasantenvironment for the beavers which also helps them find food. Forexample, the bark on the trees in the beaver pond becomes soft andedible, and there are other most things to eat in the pond.