Here is why I think Hume's account of causality is self-refuting: it presupposes a causal relation between perception and belief. Because, according to Hume, we perceive certain events constantly conjoined, we believe that they are causally related. The perception causes the belief. Hume even admits that belief in causality is very convenient, since it helps us make sense of experience. But this also entails a causal relation, since if the belief in causality helps us make sense of experience, then the belief causes us to make better sense of experience. — Noisy Calf

All that would prove is that Hume was inconsistent and could not follow his scepticism to the end. If that's your point, then I think you are right.

Bertrand Russell gave a very similar criticism, as follows:

The fact is that, where psychology is concerned, Hume allows himself to believe in causation in a sense which, in general, he condemns. Let us take an illustration. I see an apple, and expect that, if I eat it, I shall experience a certain kind of taste. According to Hume, there is no reason why I should experience this kind of taste: the law of habit explains the existence of my expectation, but does not justify it. But the law of habit is itself a causal law.
Therefore if we take Hume seriously we must say: Although in the past the sight of an apple has been conjoined with expectation of a certain kind of taste, there is no reason why it should continue to be so conjoined: perhaps the next time I see an apple I shall expect it to taste like roast beef. You may, at the moment, think this unlikely; but that is no reason for expecting that you will think it unlikely five minutes hence. If Hume's objective doctrine is right, we have no better reason for expectations in psychology than in the physical world.
(...)
Hume shrank from nothing in pursuit of theoretical consistency, but felt no impulse to make his practice conform to his theory. Hume denied the Self, and threw doubt on induction and causation. He accepted Berkeley's abolition of matter, but not the substitute that Berkeley offered in the form of God's ideas. It is true that, like Locke, he admitted no simple idea without an
antecedent impression, and no doubt he imagined an "impression" as a state of mind directly caused by something external to the mind. But he could not admit this as a definition of "impression," since he questioned the notion of "cause." I doubt whether either he or his disciples were ever clearly aware of this problem as to impressions. Obviously, on his view, an "impression" would have to be defined by some intrinsic character distinguishing it from an "idea," since it could not be defined causally. He could not therefore argue that impressions give knowledge of things external to ourselves, as had been done by Locke, and, in a modified form, by Berkeley. He should, therefore, have believed himself shut up in a solipsistic world, and ignorant of everything except his own mental states and their relations.

The fact is that, where psychology is concerned, Hume allows himself to believe in causation in a sense which, in general, he condemns.

That's exactly what I'm trying to get at. Very well put.

That's pretty well what Kant says of Hume also:

Kant argues that the understanding must provide the concepts, which are rules for identifying what is common or universal in different representations.(A 106) He says, “without sensibility no object would be given to us; and without understanding no object would be thought. Thoughts without content are empty; intuitions without concepts are blind.” (B 75) Locke’s mistake was believing that our sensible apprehensions of objects are thinkable and reveal the properties of the objects themselves. In the Analytic of Concepts section of the Critique, Kant argues that in order to think about the input from sensibility, sensations must conform to the conceptual structure that the mind has available to it. By applying concepts, the understanding takes the particulars that are given in sensation and identifies what is common and general about them. A concept of “shelter” for instance, allows me to identify what is common in particular representations of a house, a tent, and a cave.

The empiricist might object at this point by insisting that such concepts do arise from experience, raising questions about Kant’s claim that the mind brings an a priori conceptual structure to the world. Indeed, concepts like “shelter” do arise partly from experience. But Kant raises a more fundamental issue. An empirical derivation is not sufficient to explain all of our concepts. As we have seen, Hume argued, and Kant accepts, that we cannot empirically derive our concepts of causation, substance, self, identity, and so forth. The problem that Kant points out is that a Humean association of ideas already presupposes that we can conceive of identical, persistent objects that have regular, predictable, causal behavior. And being able to conceive of objects in this rich sense presupposes that the mind makes severala priori contributions. I must be able to separate the objects from each other in my sensations, and from my sensations of myself. I must be able to attribute properties to the objects. I must be able to conceive of an external world with its own course of events that is separate from the stream of perceptions in my consciousness. These components of experience cannot be found in experience because they constitute it. The mind’s a priori conceptual contribution to experience can be enumerated by a special set of concepts that make all other empirical concepts and judgments possible. These concepts cannot be experienced directly; they are only manifest as the form which particular judgments of objects take. — IEP

In other words, I take it, if Hume was correct, we couldn't actually think or make judgements; we'd have, at best, the cognitive capacity of a newborn, or someone with an acute neural disorder, because of the role of the mind in synthesising and constructing experience and sensations into a meaningful whole within which judgement is possible. And we don't naturally see that, because we see with it, or through it. That's why Kant can claim that in some basic sense, Hume must assume some of the very things that he purports to doubt. That's my gloss.

In other words, I take it, if Hume was correct, we couldn't actually think or make judgements; we'd have, at best, the cognitive capacity of a newborn, or someone with an acute neural disorder, because of the role of the mind in synthesising and constructing experience and sensations into a meaningful whole within which judgement is possible. And we don't naturally see that, because we see with it, or through it. That's why Kant can claim that in some basic sense, Hume must assume some of the very things that he purports to doubt. That's my gloss. — Wayfarer

That makes sense. If the concept of causality is necessary to intelligibly structure experience, then all experience must presuppose it in some way. And all experience includes the experience of formulating a philosophical theory, like Hume's, that attempts to explain away causal relations.

I think Kant's notion of causality might also be self-refuting, but I'll save that for a future post.

If the concept of causality is necessary to intelligibly structure experience, then all experience must presuppose it in some way — Noisy Calf

Well, all intelligible structures of experience. I suggest that A cat can wait by a mouse hole for a mouse by means of a structured experience that we can articulate intelligibly, but a the cat cannot. My take on Hume is that his concern is to draw the limits of logic and verbal reasoning. There is a widespread materialist bias that finds the idea that one cannot derive an 'ought' from an 'is' totally convincing and irrefutable, but the idea that one cannot derive a 'will be' from a 'has been' somehow contradictory.

And in neither case is Hume making an attack on the world, that there is no such thing as causation, or that there is no such thing as morality. Rather, he is making an attack on overblown rationalism that thinks it can make the world conform to thought, instead of conforming thought to the world.

Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and obey them. — Hume

This "passion" is the rock on which Humean philosophy stands, roughly translated as 'giving a damn' or 'caring'. Reason seeks to be dispassionate, but there is no reason to reason if one does not give a damn, thus passion is what is necessary to intelligibly structure experience: the cat cares about mice, and that is the primary structure of its experience prior to any reasoning it might or might not be capable of.

No, I don't think his skepticism is self-refuting. His argument seems to be that 1) we only ever observe X following from Y and that 2) it is invalid to infer from this that Y causes X.

Notwithstanding his explanation of why we believe in causation, 1 is either true or false and 2 is either true or false, and both being true is not a contradiction.

It is true that, like Locke, he admitted no simple idea without an antecedent impression, and no doubt he imagined an "impression" as a state of mind directly caused by something external to the mind. But he could not admit this as a definition of "impression," since he questioned the notion of "cause." — Russell

Hume believed that all knowledge can be categorized into "relations of ideas" and "matters of fact." On the latter he took an austere empiricist stance: matters of fact can only be known from direct sense impressions. And he argued that causation is not impressed onto us by our senses, and therefore is not a matter of fact. Neither can it be inferred by pure reason. However, he did not venture a causal theory empirical knowledge; he took the above principles as more-or-less self-evident. It is fair to dissect and question Hume's empiricist principles. It is also fair to ask whether those principles are more foundational than our causal intuitions. However, I don't think it is fair in this case to accuse him of illicitly helping himself to the very ideas that he questioned. That Hume imagined impressions being caused by their objects is a speculation.

And in neither case is Hume making an attack on the world, that there is no such thing as causation, or that there is no such thing as morality. Rather, he is making an attack on overblown rationalism that thinks it can make the world conform to thought, instead of conforming thought to the world. — unenlightened

That's one way to read him, but my impression is that he is somewhat less clear and more conflicted on this point. I'll leave this to Hume scholars though.

However, he did not venture a causal theory empirical knowledge; he took the above principles as more-or-less self-evident. It is fair to dissect and question Hume's empiricist principles. It is also fair to ask whether those principles are more foundational than our causal intuitions. However, I don't think it is fair in this case to accuse him of illicitly helping himself to the very ideas that he questioned. That Hume imagined impressions being caused by their objects is a speculation. — SophistiCat

What do you make of the rest of what Russell says then?:

I doubt whether either he or his disciples were ever clearly aware of this problem as to impressions. Obviously, on his view, an "impression" would have to be defined by some intrinsic character distinguishing it from an "idea," since it could not be defined causally. He could not therefore argue that impressions give knowledge of things external to ourselves, as had been done by Locke, and, in a modified form, by Berkeley. He should, therefore, have believed himself shut up in a solipsistic world, and ignorant of everything except his own mental states and their relations. — Russell

Here Russell elaborates further on his view (the format here got a little messed up when I copied the text, sometimes that happens when using the quote function for whatever reason, so bear with it):

Empiricism and idealism alike are faced with a problem to which, so far, philosophy has found no satisfactory solution. This is the problem of showing how we have knowledge of other things than ourself and the operations of our own mind. Locke considers this problem, but what he says is very obviously unsatisfactory. In one place we are told: "Since the mind, in all its thoughts and reasonings, hath no other immediate object but its own ideas, which it alone does or can contemplate, it is evident that our knowledge is only conversant about them." And again: "Knowledge is the perception of the agreement or disagreement of two ideas." From this it would seem to follow immediately that we cannot know of the existence of other people, or of the physical world, for these, if they exist, are not merely ideas in any mind. Each one of us, accordingly, must, so far as knowledge is concerned, be shut up in himself, and cut off from all contact with the outer world.

This, however, is a paradox, and Locke will have nothing to do with paradoxes. Accordingly, in another chapter, he sets forth a different theory, quite inconsistent with the earlier one. We have, he tells us, three kinds of knowledge of real existence. Our knowledge of our own existence is intuitive, our knowledge of God's existence is demonstrative, and our knowledge of things present to sense is sensitive (Book IV, Ch. III).
In the next chapter, he becomes more or less aware of the inconsistency. He suggests that some one might say: "If knowledge consists in agreement of ideas, the enthusiast and the sober man are on a level." He replies: "Not so where ideas agree with things." He proceeds to argue that all simple ideas must agree with things, since "the mind, as has been showed, can by no means make to itself" any simple ideas, these being all "the product of things operating on the mind in a natural way." And as regards complex ideas of substances, "all our complex ideas of them must be such, and such only, as are made up of such simple ones as have been discovered to coexist in nature."

Again, we can have no knowledge except (1) by intuition, (2) by reason, examining the agreement or disagreement of two ideas, (3) "by sensation, perceiving the existence of particular things"
(Book IV, Ch. III, Sec. 2).

In all this, Locke assumes it known that certain mental occurrences, which he calls sensations, have causes outside themselves, and that these causes, at least to some extent and in certain respects, resemble the sensations which are their effects. But how, consistently with the principles of empiricism, is this to be known? We experience the sensations, but not their causes; our experience will be exactly the same if our sensations arise spontaneously. The belief that sensations have causes, and still more the belief that they resemble their causes, is one which, if maintained, must be maintained on grounds wholly independent of experience. The view that "knowledge is the perception of the agreement or disagreement of two ideas" is the one that Locke is entitled to, and his escape from the paradoxes that it entails is effected by means of an inconsistency so gross that only his resolute adherence to common sense could have made him blind to it.

This difficulty has troubled empiricism down to the present day. Hume got rid of it by dropping the assumption that sensations have external causes, but even he retained this assumption whenever he forgot his own principles, which was very often. His fundamental maxim, "no idea without an antecedent impression," which he takes over from Locke, is only plausible so long as we think of impressions as having outside causes, which the very word "impression" irresistibly suggests. And at the moments when Hume achieves some degree of consistency he is wildly paradoxical.

Also, regardless of how one views Hume's definition of an impression, it is clear that the law of habit is a causal law, and therefore Hume had no right to use it as an explanation for how the ideas of cause and effect come about, except as a sort of “reductio ad absurdum”, showing where seemingly good reasoning with sound principles has led him.

Also, regardless of how one views Hume's definition of an impression, it is clear that the law of habit is a causal law, and therefore Hume had no right to use it as an explanation for how the ideas of cause and effect come about, except as a sort of “reductio as absurdum”, showing where seemingly good reasoning with sound principles has led him. — Amalac

Habit is not a law at all. It is my habit to drink coffee for breakfast. But habit is not the cause of my drinking coffee, it is the mere fact that I do. Sometimes I might I have tea instead, and no law is broken, only my habit.

(...)the law of habit is itself a causal law.
Therefore if we take Hume seriously we must say: Although in the past the sight of an apple has been conjoined with expectation of a certain kind of taste, there is no reason why it should continue to be so conjoined: perhaps the next time I see an apple I shall expect it to taste like roast beef. You may, at the moment, think this unlikely; but that is no reason for expecting that you will think it unlikely five minutes hence. If Hume's objective doctrine is right, we have no better reason for expectations in psychology than in the physical world.

Do you disagree with any of that?

By the way, I said “the law of habit” not “habit”, they are not quite the same thing.

Yes. It's nonsense though and an abuse of the notion of habit. Birds have a habit of singing in the morning. There may be a cause or many causes or none, but the singing is not caused by the habit of singing. It makes no sense at all. Even the biggest cheeses are sometimes a bit off.

Birds have a habit of singing in the morning. There may be a cause or many causes or none, but the singing is not caused by the habit of singing. — unenlightened

That’s not what Russell is saying, he’s saying that my expectation that when I see an apple in the future, it will taste like how apples usually taste and not like roast beef, is explained by the fact that I have always expected apples to taste that way in the past whenever I see them.

Thus, although in the past the sight of an apple (cause) has been conjoined with expectation of a certain kind of taste (effect), that gives no justification for the claim that it will continue to be so conjoined in the future.

Basically, expectations also fall prey to the problem of induction.

Habit is not a law at all. It is my habit to drink coffee for breakfast. But habit is not the cause of my drinking coffee, it is the mere fact that I do. Sometimes I might I have tea instead, and no law is broken, only my habit. — unenlightened

However in the case of expecting causation and the world to go on being predictable in the future, the habit is always there. So it's not like usually drinking coffee in the morning. It's expecting that there will be such a thing as coffee to drink, which will have a certain flavor and caffein content that has some stimulating effect on your nervous system.

Thus, although in the past the sight of an apple (cause) has been conjoined with expectation of a certain kind of taste (effect), that gives no justification for the claim that it will continue to be so conjoined in the future. — Amalac

Yes, the expectation is not justified. That's why it's just a habit and not a reason. That there is no rational justification is what Hume is saying. We expect the future to be like the past because NO REASON, we just do it habitually. What is daft is to claim that Hume needs to justify his habits when he's just said there is no justification for them.

It's expecting that there will be such a thing as coffee to drink, which will have a certain flavor and caffein content that has some stimulating effect on your nervous system. — Marchesk

And there is no justification for it. That is what the man says. We do it, and reason cannot justify it.

And there is no justification for it. That is what the man says. We do it, and reason cannot justify it. — unenlightened

No wonder Kant was worried about science after Hume. If we take Hume seriously, there is no reason for anything that happens. Therefore, there are no explanations. Just descriptions of constant conjunction, up to this point. Science is a fancy kind of book keeping.

My constantly conjoined habit results in me wondering why there is such a vast constant conjunction of events throughout the observable universe. It being radically contingent like that beggars belief. So does the thought that it could change at any moment, for no reason.

My habit means I can't buy Humean skepticism.

Kant was just trying to save the idea of rationality as an adequate response to the world. There is no problem for science at all. Science laws are rephrased as descriptive rather than predictive, and then good old habit just carries on as before. The only casualty is the idea of 'man, the rational being'.

Science laws are rephrased as descriptive rather than predictive, and then good old habit just carries on as before. The only casualty is the idea of 'man, the rational being'. — unenlightened

I disagree, since theories need to also be predictive. That and every scientist talks in terms of explanation and cause when they're not waxing philosophical.

Yes, the expectation is not justified. — unenlightened

It's not just that we may be wrong about how future apples will taste, but also that we have no reason to expect that in the future we will even expect that they will have their usual taste instead of expecting that they will taste like ice cream.

I do not wish, at the moment, to discuss induction, which is a large and difficult subject; for the moment, I am content to observe that, if the first half of Hume's doctrine is admitted, the rejection of induction makes all expectation as to the future irrational, even the expectation that we shall continue to feel expectations. I do not mean merely that our expectations may be mistaken; that, in any case, must be admitted. I mean that, taking even our firmest expectations, such as that the sun will rise
tomorrow, there is not a shadow of a reason for supposing them more likely to be verified than not. — Russell

That is to say: if Hume is right, then not only is our expectation that the sun will rise tomorrow not justified, but neither is the expectation that we will continue to expect the sun to rise tomorrow justified.

If that's the case, Hume has no right to say such things as:

Nature, by an absolute and uncontrollable necessity has determined us to judge as well as to breathe and feel; nor can we any more forbear viewing certain objects in a stronger and fuller light, upon account of their customary connexion with a present impression, than we can hinder ourselves from thinking as long as we are awake, or seeing the surrounding bodies, when we turn our eyes towards them in broad sunshine. Whoever has taken the pains to refute this total scepticism, has really disputed without an antagonist, and endeavoured by arguments to establish a faculty, which nature has antecedently implanted in the mind, and rendered unavoidable. My intention then in displaying so carefully the arguments of that fantastic sect, is only to make the reader sensible of the truth of my hypothesis, that all our reasonings concerning causes and effects are derived from nothing but custom; and that belief is more properly an act of the sensitive, than of the cogitative part of our natures.

Hume has no right to say such things as: — Amalac

]Whoever has taken the pains to refute this total scepticism, has really disputed without an antagonist, — Hume

Do you not see that he is talking exactly about the argument you are making? He agrees with you that he has no right to say such things, to the extent that you mean he has no justification in reason. However, everybody does think, say and presume such things, himself included.

That is to say: if Hume is right, then not only is our expectation that the sun will rise tomorrow not justified, but neither is the expectation that we will continue to expect the sun to rise tomorrow justified. — Amalac

So what you're saying is that we have no reason to think we'll have a habit of expecting things in the future. If there is no reason for constant conjunction, that includes our habits in the future.

This sounds like Hume's reason has gone awry. The way out is just to admit there must be some causal factor at play in the world, even if we can't observe it. And science does assume this when it includes unobservables in it's theories. If there was no reason for any conjunction, then there's no need to posit unobservables.

That's what Russell is saying would be the case if Hume was right (if I understood him correctly).

The way out is just to admit there must be some causal factor at play in the world, even if we can't observe it. And science does assume this when it includes unobservables in it's theories. If there was no reason for any conjunction, then there's no need to posit unobservables. — Marchesk

As I said at the outset and @Amalac's quote above confirms, Hume is not trying to deny causation any more than he is trying to deny morality. He is showing the limits of reason. It is certainly unreasonable to demand that he exceed the limits he is trying to establish.

I think Hume's account of causality is self-refuting: — Noisy Calf

Any philosophical approach that utilizes a notion of refutation courts skepticism.

And in neither case is Hume making an attack on the world, that there is no such thing as causation, or that there is no such thing as morality. Rather, he is making an attack on overblown rationalism that thinks it can make the world conform to thought, instead of conforming thought to the world. — unenlightened

If that's all Hume were trying to do, then I would happily get behind him. But I'll leave the discussion of Hume's belief system to others. Unfortunately for people like me who enjoy speculation more than research, an ism that takes its name from a philosopher is probably less nuanced and compelling than the ideas of the philosopher it's named after.

No, I don't think his skepticism is self-refuting. His argument seems to be that 1) we only ever observe X following from Y and that 2) it is invalid to infer from this that Y causes X. — Michael

I agree that those two premises are not refuted, at least not directly, by the conclusion that we have no knowledge of causality. But as you seem to admit, the conclusion does refute Hume's (apparent?) notion that we can reasonably identify constant conjunction as the source of our belief in causality and our expectations regarding future events. And in the the third and fourth paragraphs of my original post, I argued that a form of causal skepticism that doesn't offer an explanation for our causal beliefs is explanatorily inadequate, and should thus be rejected as unsound. Any belief that makes it impossible to explain key facts about reality, I think, ought to be rejected.

And maybe the argument you presented is indirectly refuted by non-explanatory causal skepticism. For if we don't have a way of explaining why we believe some things to cause other things, we can't rationally distinguish between causal and non-causal relations. Thus, we cannot justify believing in any causal relations, whether these causal relations are real and physical or merely intuitive and psychological. But the idea of knowledge seems to presuppose at least some form of causal relation. For the causal realist, the mental act of justification really causes a belief to become an item of knowledge. For the Humean skeptic, the fact that the mental act of justification always precedes beliefs becoming more firmly held suffices to differentiate, at least from a psychological standpoint, mere belief from what we call knowledge. But for the non-explanatory skeptic, who denies constant conjunction as an explanation for our causal intuitions, nothing seems to differentiate knowledge from unjustified belief. Therefore, if non-explanatory skepticism is true, we can have no reason to believe either of the two premises you presented. Can you find a flaw in this argument? If not, then perhaps causal skepticism is self-refuting whether or not it tries to explain causal beliefs, although non-explanatory causal skepticism refutes itself in a less direct way.

However, I don't think it is fair in this case to accuse him of illicitly helping himself to the very ideas that he questioned. That Hume imagined impressions being caused by their objects is a speculation. — SophistiCat

I seem to remember Hume illicitly helping himself to causal notions, but perhaps my memory is foggy and or my reading was uncharitable. But others in this thread seem to be debating exactly what Hume meant, so I will leave that to them.

Any philosophical approach that utilizes a notion of refutation is a form of skepticism. — Joshs

Are you saying that philosophical skeptics always employ a kind of self-destruct mechanism that they believe is rooted within the structure of human reason? For instance, skeptics will often point to paradoxes as evidence that reason cannot offer a consistent explanation reality. Is Hume, by showing that belief in causation is unreasonable, showing that any kind of belief, including belief that belief in causation is unreasonable, is unreasonable? This reminds me of the Wittgenstein quote about the ladder which, once it gets you to the final conclusion, you must kick away. Once reason succeeds at undermining itself, you can just stop using reason, so you don't need reason to justify your skepticism. Is this what you're getting at?

And in the the third and fourth paragraphs of my original post, I argued that a form of causal skepticism that doesn't offer an explanation for our causal beliefs is explanatorily inadequate, and should thus be rejected as unsound. Any belief that makes it impossible to explain key facts about reality, I think, ought to be rejected. — Noisy Calf

It may be that there is no such thing as causation, or that causation is inexplicable, or that there is an as-of-yet unknown means to explain causation. If one of these is true then I don’t think Hume’s argument fails.

After all, I don’t need to provide an alternative to a God of the gaps to argue that a God of the gaps explanation is either false or unjustified.

It may be that there is no such thing as causation, or that causation is inexplicable, or that there is an as-of-yet unknown means to explain causation. If one of these is true then I don’t think Hume’s argument fails. — Michael

In the original post and in my response to you, I differentiated Hume's causal skepticism (some in the thread have argued that Russel and I are wrong to attribute this to Hume, but so be it), which self-refutingly explains our causal beliefs by appealing to constant conjunction, and non-explanatory causal skepticism, which doesn't attempt to explain our causal beliefs at all. And I made two arguments 1) non-explanatory causal skepticism fails to explain key facts about reality 2) non-explanatory causal-skepticism is specifically necessary to account for human knowledge. By key facts about reality, I just mean things like why are cheetahs faster than dogs, or why do people slip on wet floors, or why is it dangerous to drive blindfolded. Without at least some vague notion of causality, I don't think you could answer any of these questions.

As for 2), why non-explanatory causal-skepticism is specifically necessary to account for human knowledge, refer to the arguments in my previous posts. If there is no causation, I argued, there can be no knowledge. So if causation is inexplicable, then knowledge is inexplicable, and we can't explain the difference between justified knowledge and unjustified belief. If there is an as-of-yet unknown means to explain causation, then the means of explaining what makes some beliefs justified and others not justified is also as-of-yet unknown, so we cannot know the difference between knowledge and unjustified belief. All three of these conclusions would invalidate all of our knowledge and lead to radical skepticism. For if we can't know anything, we have no justification to belief anything. If radical skepticism is true, then philosophy is futile. So I think radical skepticism is also self-refuting, since it can't justify itself.

After all, I don’t need to provide an alternative to a God of the gaps to argue that a God of the gaps explanation is either false or unjustified. — Michael

I think that depends on what you mean by God of the gaps. I think there are valid and invalid gap arguments. An invalid gap argument would go like this: 1) P is a fact that we have no explanation for, 2) if Q were true, Q would explain P, 3) therefore, Q is true. But a valid gap argument would be: 1) either P is true or Q is necessarily inexplicable, 2) If Q is necessarily inexplicable, then it is impossible to provide a coherent account reality, 3) therefore P is true. My argument is of the latter kind, not of the former kind: 1) Either it can be explained why certain things are causally related, or human knowledge is necessarily inexplicable, 2) if human knowledge is necessarily inexplicable, then it is impossible to provide a coherent account of reality, and radical skepticism ensues, 3) therefore, it can be explained why certain things are causally related, namely, why the mental act of justification causes belief to become knowledge.

Also, regardless of how one views Hume's definition of an impression, it is clear that the law of habit is a causal law, — Amalac

This is not true, I don't think so. The law of habit is not a law of causation. It is simply a recurring occurrence, much like every other law has recurring occurrences that support that particular law. The fact that people see laws where there are no laws, can be due to a recurring coincidence or people seeing laws where there are none. There is no self-contradiction there.

I know the consensus is there, but in my humble opinion, the consensus is wrong.

Hume's opinion is not a refutation of the universe of causation and determinism; it is a parallel explanation to it. You don't need to believe it, but you have to accept, that it's a valid way of looking at things.

Hume's world of coincidences, and the world viewed as a series of causations, are both valid, but mutually exclusive.

This is not true, I don't think so. The law of habit is not a law of causation. — god must be atheist

At the risk of making you lose your patience again... isn’t this relation between the sight of an apple and the expectation of a certain kind of taste, an instance of the causal law of habit?:

That’s not what Russell is saying, he’s saying that my expectation that when I see an apple in the future, it will taste like how apples usually taste and not like roast beef, is explained by the fact that I have always expected apples to taste that way in the past whenever I see them.

Thus, although in the past the sight of an apple (cause) has been conjoined with expectation of a certain kind of taste (effect), that gives no justification for the claim that it will continue to be so conjoined in the future.

Basically, expectations also fall prey to the problem of induction. — Amalac

We expect that we will expect apples to taste how they usually taste, because we are habituated to think that way, and so we expect that we won’t cease to expect them to taste like they usually taste, and not like ice cream in, say, the next 5 minutes. But even the truth of this claim depends on the validity of induction from particular instances, and so it is not even founded on probability:

if we take Hume seriously we must say: Although in the past the sight of an apple has been conjoined with expectation of a certain kind of taste, there is no reason why it should continue to be so conjoined: perhaps the next time I see an apple I shall expect it to taste like roast beef. You may, at the moment, think this unlikely; but that is no reason for expecting that you will think it unlikely five minutes hence. If Hume's objective doctrine is right, we have no better reason for expectations in psychology than in the physical world. — Russell