Everyone who studies a given philosopher, or, I suppose, just about anything, has their own mental laundry list of clean-up points, by which I mean the set of common misunderstandings that, come what may, you are going to eliminate. Of course, this is optimistic thinking, since the laundry list gets longer and longer as time goes on, but something about those misunderstandings puts you on a mission to eliminate them.
One of the entries on my laundry list is the so-called 'in-principle argument' in Part I of Hume's essay on Miracles. This argument has haunted interpretation of Hume like the Flying Dutchman has haunted the Seven Seas; it is a ghost argument. In the history of philosophy you run across such ghost arguments from time to time: people begin seeing arguments in texts that are not there, but to someone reading them with certain presuppositions (not least of which is the mere expectation of finding the argument there), it looks like it is there. In the case of long-term hauntings this is often due to some quirk of the text that makes the real argument difficult to understand, and, as the ghost argument usually makes sense (even if it is rejected), it becomes easy to see things in the text that look like the ghost, and to ignore things in the text that are inconsistent with it, sometimes even explicitly so. This is precisely the case with the 'in-principle argument'.
The essay on Miracles falls into two parts. Much of Part II is fairly easy to follow; it consists of a number of a posteriori arguments against testimony for miracles, empirical evidences for regarding it as unreliable. Part I, however, is not so obvious. Its explicit conclusion is a rather puzzling play on words: "no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavors to establish; and even in that case there is a mutual destruction of arguments, and the superior only gives us an assurance suitable to that degree of force, which remains, after deducting the inferior." It appeals to a complicated psychological mechanism, talks of proofs balancing proofs, and has many more puzzling characteristics. But Hume does say that there is "a uniform experience against every miraculous event" and that "as a uniform experience amounts to a proof, there is here a direct and full proof, from the nature of the fact, against the existence of any miracle." That sounds pretty critical of miracles, and we know in any case from Part II that Hume is not impressed by testimony for miracles, and so people have naturally taken Part I to be an argument against miracles. Moreover, since there is a uniform experience against every miracle, and such uniform experience is a "direct and full proof" against the existence of any miracle, what can be more natural than reading Part I as an argument that testimony of miracles should be rejected in principle. It has the advantage of giving the essay a pleasing symmetry: Part I is the in-principle argument, Part II is, so to speak, the in-practice argument. It's a two-pronged attack against testimony for miracles, and looks quite clever. It's not surprising that it keeps appearing when people read the text.
But the text clearly cannot bear this interpretation. Part I has no in-principle argument against miracles. In fact, it has no argument against miracles at all. It's the set-up for the real argument, which is found in Part II. When Hume tells us that uniform experience is a "direct and full proof" against the existence of any miracle, he does not leave it at that. He immediately goes on to say, "nor can such a proof be destroyed, or the miracle rendered credible, but by an opposite proof, which is superior." And in the paragraph prior to this one, he tells us what he is doing:
But in order to encrease the probability against the testimony of witnesses, let us suppose, that the fact, which they affirm, instead of being only marvellous, is really miraculous; and suppose also, that the testimony considered apart and in itself, amounts to an entire proof; in that case, there is proof against proof, of which the strongest must prevail, but still with a diminution of its force, in proportion to that of its antagonist.
Proof against proof! Hume has set us up in a situation in which we have a "direct and full proof" that miracles don't happen, and an "entire proof" that at least one has. So, he says, we are to take subtract the force of the weaker proof from the force of the stronger proof, and accept the conclusion of the stronger proof with whatever force it has left after the subtraction. We don't think of proofs as capable of having opposites; but Hume does. We don't think of "entire proofs" or "full proofs" as coming in degrees; but Hume does. If you have any doubts about this, I give you Hume's own words, in his response to Campbell's attack on him on this point:
I find no difficulty to explain my meaning, and yet shall not probably do it in any future edition. The proof against a miracle, as it is founded on invariable experience, is of that species or kind of proof, which is full and certain when taken alone, because it implies no doubt, as is the case with all probabilities; but there are degrees of this species, and when a weaker proof is opposed to a stronger it is overcome.
Deciding not to explain this point in future editions was perhaps a mistake, because it has often been overlooked. In Part I Hume explicitly supposes that we have a full proof for the miracle; and the conclusion he draws is not that testimony for a miracle should be rejected in principle, but that it should be rejected unless it is a sufficiently strong proof to override the proof of the laws of nature. Recognizing this we understand what Hume is really doing in Part II: he is arguing that, in fact, no testimony for religious miracles ever attains to this standard. And this is what he explicitly says he is doing, right at the beginning of Part II:
In the foregoing reasoning we have supposed, that the testimony, upon which a miracle is founded, may possibly amount to an entire proof, and that the falsehood of that testimony would be a real prodigy: But it is easy to shew, that we have been a great deal too liberal in our concession, and that there never was a miraculous event established on so full an evidence.
So there is no in-principle argument against miracles in Part I of the essay, indeed, no argument against miracles at all; he is simply setting up the argument in Part II. It's easy to see why the ghost haunts interpretations of the text, but it is a ghost. It is time that we finally salted that grave.