Moral agency further implies, that we are accountable for our conduct, and that if we do what we ought not to do, we deserve blame and punishment. My conscience tells me, that I am accountable for those actions only that are in my own power; and neither blames nor approves, in myself or in others, that conduct which is the effect, not of choice, but of necessity. Convince me, that all my actions are equally necessary, and you silence my conscience for ever, or at least prove it to be a fallacious and impertinent monitor: you will then convince me, that all circumspection is unnecessary, and all remorse absurd. And is it a matter of little moment, whether I believe my moral feelings authentic and true, or equivocal and fallacious? Can any principle be of more fatal consequence to me, or to society, than to believe that the dictates of conscience are false, unreasonable, or insignificant? Yet this is one certain effect of my becoming a Fatalist, or even a sceptic in regard to moral liberty.
I observe, that when a man's understanding begins to be so far perverted by debauchery, as to make him imagine his crimes unavoidable, from that moment he begins to think them innocent, and deems it a sufficient apology, that in respect of them he is no longer a free, but a necessary agent. The drunkard pleads his constitution, the blasphemer urges the invincible force of habit, and the sensualist would have us believe, that his appetites are too strong to be resisted. Suppose all men so far perverted as to argue in the same manner with regard to crimes of every kind;--then it is certain, that all men would be equally disposed to think all crimes innnocent.
(By 'Fatalism' Beattie means what we would call 'determinism'.) Much of Beattie's argument on liberty and necessity is difficult to follow, in part because it seems to confuse various senses of 'liberty'. Or, indeed, it may be quite deliberate; he may be refusing to distinguish (sharply) these various senses, preferring to treat them as all intimately connected. Whatever's the case with the rest of the argument, however, I think the above argument particularly interesting, because it seems to me to highlight a problem people have with determinism that determinists have not done much to alleviate. The idea is this:
1. If the necessity proposed by the determinist is the same as that to which the debauchee appeals to excuse himself, determinism makes moral responsibility impossible.
2. But if it is a different sort of necessity, as it needs to be the case to preserve the whole notion of moral responsibility, what sort of necessity would that be? And how would it be distinguished from the necessity that would excuse?
This isn't, of course, a refutation of determinism. But it is a challenge to those who hold that determinism is compatible with moral responsibility, a challenge to give some principled reason for distinguishing the necessity of determinism from the necessity of the excuse of the debauchee. We need a reason, if we are to preserve moral responsibility, to think determinism does not give the latter. This sharpens one aspect of the big problem determinists have always had to deal with, namely, the question of whether determinism is actually compatible with morality at all. This problem of the two necessities, then, is one of the fronts on which determinists have to fight.
As I said, I haven't really seen anything that shows determinists making any great effort to distinguish the two necessities. A common argument for determinism, in fact, going back to the early modern period, appears to require that there be only one necessity in the two cases, namely, the argument that determinism is necessary for making sense of moral character and causation. But if the two are the same, then determinism actually eliminates moral considerations entirely, so the determinist needs some significant difference in the cases.