According to Hume, causality cannot be found in "things themselves", nor can it be empirically accessible. Instead, it is we, the observers, who attach causal relations among things merely because we constantly see them occurring together, i.e. merely due to Habit.

Kant was convinced by Hume's argument that causality is not an inherent feature of things themselves. However, he deeply refused to accept that it is only a matter of habit, and, in order to "escape" out of the conundrum, was forced to create an entire new (and quite fruitful) field of philosophy. The fact that he had to do that much stretching and make his famous "Copernican Turn" in order to "solve the problem" just goes to show how much Hume's position bothered him.

Now, the question is: why was Kant so bothered about causality being merely a habit? What consequences did Kant see in Hume's statement that forced him to revolutionise philosophy just to refute it? Surely we can still have knowledge and science even if we say that causality is not valid a priori, no?

EDIT: After giving some more thought to this, I start to get the gist of how problematic Hume's position can be for science.

A key step in advancing our knowledge is when an observation is made that contradicts our current theory. The scientist then has two choices:

1) Assume causality holds, and immediately look for causes of this discrepancy. Because of the Principle of Uniformity, one has to assume that this phenomenon has always been present in Nature, but it was simply neglected by our previous theory. The new theory then has to explain how the previous one worked so well, even without taking these phenomena into account. I.e. the previous theory must somehow be "included" in the new theory. This necessarily leads to a broadening of our understanding and of our control over the world.

2) Take a skeptical position about causality, which means these new observations can be merely a consequence of causality breakdown, and we can say nothing more about them. Our previous theories remain correct to the best of what we can possibly say. Whenever they are contradicted by experiments, we have nothing to do but to frustratingly accept that Nature can play tricks on us.

The Humean viewpoint does not even allow to investigate a cause of this purported breakdown in causality: one really can say nothing about it.

Of course, even a partisan of Hume's side can take position (1) "for practical purposes", but by doing so he is admitting that one needs to assume causality a priori in order to have knowledge of the world.

In conclusion, in order to at least have the possibility of knowledge, one has to take the standpoint that the world is necessarily logical.

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    Because he felt that the same line of reasoning would make even mathematical knowledge (in Kant's demanding sense) impossible. If Hume considered that, reasoned Kant, "his good sense would have stopped him". Hume did consider that (in works Kant apparently did not read) and was unimpressed, but such pragmatic devaluation of Knowledge was unacceptable to Kant. See Kant and Hume on Causality. – Conifold Feb 23 '17 at 23:34
  • I have been reading this link for the past few days already but, as far as I got, there is no discussion as to why Kant was so annoyed about Hume's point. You mention a "pragmatic devaluation of knowledge". But how is Hume's position a devaluation? IIf one shows that, without an a priori causality, knowledge would be less solid or even impossible, then I would understand Kant. But I just fail to see it. I think your comment on mathematics may shed some light into it. Although it would only mean that maths has also an empirical aspect to it? There is, in principle, no problem with that. – tchuncly Feb 24 '17 at 0:06
  • The skepticism of Hume remains attached to perception and does not justify the generality and necessity but explains these as subjective coincidence and habit. (it stays at Perception and Reason is unjustified) The critical philosophy of Kant corrects this by locating the elements (categories) that belong to the self-acting of cognition and so exist a priori and are used so to transform perception to experience. – John Am Feb 24 '17 at 0:16
  • @JohnAm I think I understand the difference between both thinkers. The question is: is there some fundamental reason to abhor Hume's empiricism, or is it just a caprice of someone who doesn't want the metaphysical categories to be empirical? Suppose I am a Kantian, or a German Idealist in general, and I am faced with a Humean. How do I convince him that he is "wrong", or that my philosophy is more consistent than his? Surely I can't just say "I don't like the categories to be empirical, therefore they must exist a priori". – tchuncly Feb 24 '17 at 0:40
  • It may seem like a caprice today, but pretty much all classical rationalists before and after Kant (from Plato to Husserl) believed that only that which comes with absolute necessity and "apodictic certainty" is "true knowledge". Examples of mathematics and mathematical physics were very powerful, that they were not all that was implausible to them, and they became the prototypical examples for knowledge in general. The question was only how to justify it. Only after the demise of classical physics and splintering of classical mathematics in 1900s Humean position gained credibility. – Conifold Feb 24 '17 at 2:37

There is a bit of confusion in the question, that needs to be cleared up first. You start by noting, correctly, that Hume argued that causation is based on habit, that is a subjective basis. Yet you end by attributing to him, incorrectly, the claim that causality is (merely) not valid a priori. Hume's conclusion was rather that causation judgments are not valid at all. That is, not having a rational justification of any kind.

Hume was a skeptic. His assumptions were empiricist, but his conclusions were skeptical. Kant saw Hume as the best exemplar of an empiricist. Accordingly, Kant always identifies empiricism with skepticism. And this, I think, is the main reason why Kant "refused" to accept Hume's account of causation. The choice here was not between a priori and a posteriori justifications. The choice was between having and not having (and giving up on) justification at all.

Hume is perhaps the ablest and most ingenious of all sceptical philosophers ... To the uncritical dogmatist, who has not surveyed the sphere of his understanding ... these attacks of scepticism are not only dangerous, but destructive ... And thus scepticism, the bane of dogmatical philosophy, conducts us to a sound investigation into the understanding and the reason. When we are thus far advanced, we need fear no further attacks. (Critique of Pure Reason "The Discipline of Pure Reason in Polemics")

In addition, regarding the concept of causation in particular, Kant believed that causation without some sort of objective necessity was just not causation at all, so that accepting Hume's account would amount to giving up on the concept of causation entirely.

Indeed, the conception of a cause so plainly involves the conception of a necessity of connection with an effect, and of a strict universality of the law, that the very notion of a cause would entirely disappear, were we to derive it, like Hume, from a frequent association of what happens with that which precedes; and the habit thence originating of connecting representations — the necessity inherent in the judgement being therefore merely subjective. (Critique of Pure Reason Introduction)

Kant born about when Newton died, and was deeply invested in science. He needed a way to interpret the effectiveness of the new sciences' counterintuitive abstractions.

Is propagating our genes just a habit, or it is something deeper? Because interpreting causality is a strategy for survival. And animals take survival dead earnest, in a way we presume arises before habit.

I think the most reasonable way of looking at Kant's underlying idealism these days, if you are not positively disposed toward idealism proper, is in terms of genetic forces. It is hard to imagine a world where science works as well as it does, and historical forces push us toward making it more and more effective, while it makes less and less intuitive sense, if causation is simply habitual.

Habit would generate a science that fits our naive minds. Aristotle would be adequate. We would never get to Newton. So if you believe in scientific progress, there has to be a layer of investment in causation deeper than simple continuity or conformity. There has to be some force that presses us into that habit.

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