Hume applying rationality to causality showed that it could not exist - the question is then how does it exist. Kants reply was to place it in the mind. Popper discusses this in his *Conjectures & Refutations
Thus Kant’s reply to Hume came near to being right; for the distinction between an a priori valid expectation and one which is both genetically and logically prior to observation, but not a priori valid, is really somewhat subtle.
So Popper appears to agree with Kant here.
But Kant proved too much. In trying to show how knowledge is possible, he proposed a theory which had the unavoidable consequence that our quest for knowledge must necessarily succeed, which is clearly mistaken.
What does he mean by this? I read this first meaning the quest for all knowledge; but this cannot be right; I suspect given his theory of falsifiability one cannot attain proven knowledge of the world - there can always be new evidence that fails it. However, given Kants starting point I think Kant means knowledge of a more basic kind - measuring a metre, measuring a second, that there are two bottles there; or taking that this has caused that. But perhaps, and probably more likely, he means more. What was Kants 'quest for knowledge'?
When Kant said "Our intellect does not draw its laws from nature but imposes its laws upon nature", he was right.
Again he agrees with Kant on how nature is ordered by the mind.
But in thinking that these laws are necessarily true, or that we necessarily succeed in imposing them upon nature, he was wrong. Nature very often resists quite successfully, forcing us to discard our laws as refuted; but if we live we may try again.
But then what does he mean by this? If Nature 'resists' our laws forcing us to 'discard' them; then it appears he is describing the scientific laws - those of chemistry or physics, say. Surely he cannot mean the laws grounding experience - those of space, time, causality & quantity?
One might suppose that these laws are distorted in the hallucinations or dreams - but I suspect that these states of the mind is not what Kant was discussing; that is he took it as implicitly read he was talking about the human conscious when it is awake and functioning normally.
Kant believed that Newton’s dynamics was a priori valid (See his Metaphysical Foundations of Natural Science, published between the first and the second editions of the Critique of Pure Reason.) But if, as he thought, we can explain the validity of Newton’s theory by the fact that our intellect imposes its laws upon nature, it follows, I think, that our intellect must succeed in this; which makes it hard to understand why a priori knowledge such as Newton’s should be so hard to come by.
Is Popper just grumbling here - or is his attack serious?