Let's deal with Newton first. I've spent some time with the Principia recently, helping proof-read a new translation, so I feel I've got some insight into his methods. First and foremost, the Principia as it is presented to the reader is almost entirely without recourse to empirical concepts. Rather than determine that the planets move in ellipses, and comets in hyperbolae, and then determining the force that must govern them, Newton determines any number of possible forces (ones that diminish at a steady rate with distance, ones that diminish with the square of the distance, ones that diminish with the cube of the distance...etc.) and only after demonstrating geometrically (and with his amazing tool, calculus), does he take note of what the planets are actually doing. While the important curves are the empirical ones, he finds them a priori. This technique allows him a certainty that is unavailable to empiricists.
Poor maligned Kant. And poor maligned idealism. I mean, the idea seems simple: you don't perceive ultra-violet, right? So your visual perceptions don't take it into account. These perceptions are, in a sense, conditioned by the limits of our visual spectrum.
Now, take your brain. Why would we think that our thoughts are not similarly limited and conditioned? That's all idealism is saying: that the perceptions and thoughts which we have are not the raw stuff of truth, but that we add to and shape those perceptions and thoughts ourselves.
Kant finds no evidence for causality in nature, but also rejects the stance Hume took, which claimed that causality is established in our minds by seeing two events take place, one always after the other. By Hume's point of view, then, I should have felt that the waving of sticks and the setting up of a bamboo airport really would have brought down planes.
Kant rightly rejects this view, insisting that causality is in fact imposed by us on our thoughts. Does this mean that we can simply turn it off, and be absolved of all our misdeeds? No, because causality is a priori. Now, I use a priori to mean prior to experience, but Kant has rather a more comprehensive take. For him, the a priori realm contains all that is prior to experience, universal and necessary. Universal and necessary? Yep, which means that, a) everybody sees things the same way (he has good reasons for feeling so, which I won't go into), and b) they can't help it. So no turning off your own personal causality when it's inconvenient.
Causality is only one example of the categories of thought. I don't see a need to go into the others. They are simply the things that we assume, and cannot help assuming, in every thought we have. Our perceptions are conditioned similarly by the a priori, which in their case is space and time.
Kant would also be the last to devalue experience: he felt that all knowledge was found empirically. But he also knew that that knowledge was conditioned by the a priori categories of thought we have. These a priori categories are little different from the sort of "hard-wiring" of the brain that empiricists claim for us; why be so down on them? Because a philosophy is never what influences people; it's the myths about that philosophy that they absorb. With idealism, it was the myth that idealism decreed that the universe was "all in our heads", and therefore we go somehow either alter it with our thoughts or know everything without experience. As I said above, nothing could be further from Kant's view.
UPDATE:
Outraged shrieks, eh? That's what I get for posting a measured, researched response. Next time I'll just resort to the ol' ad hominem.
UPDATE II: Mr. Thomas of One Good Turn comments. I can understand someone who's thought these issues over, as Mr. den Beste clearly has, not wanting to discuss something he considers obvious. I disagree, but you knew that already.
