There have been philosophers that have had extraordinary minds, as demonstrated by their writings. But despite their great intellects we also have rejected many of their central ideas, although we do keep bits and pieces as impossible to improve upon, and much modern philosophy is done in response to their thoughts, and not built upon them. A great example of this is Kant. I don’t think any modern philosopher accepts Kant as wholly correct. Different people take different ideas away from Kant, but argue for them in ways that Kant did not, and often modify them extensively. Consider the categorical imperative. Its simplicity and basis in rationality make it seem attractive to many, but it is obvious that it has serious flaws. Not all situations can be appropriately generalized, and occasionally the categorical imperative would endorse behavior that is clearly unethical. But Kant was no fool, why didn’t he see these problems?
I think the problem is rooted in the attempt to do philosophy based on some set of “first principles”, and to solve all problems with a philosophy developed from those principles. This is not an approach attempted by many modern philosophers, but in previous centuries many of the great minds seemed to do philosophy in this way, perhaps starting with Aristotle. And there are obvious benefits to this approach. For one, if done correctly, it becomes hard to argue with such a philosophy, because to do so would seemingly be to reject those indubitable first principles (and I think this is the primary motivation). There is also the added benefit that such philosophies tend to give rise to schools (groups) of people who follow that philosophy, which is certainly flattering to the philosopher’s ego (or would be if these groups didn’t tend to arise after the philosopher’s death).
Certainly the promise of a philosophy that cannot be disputed is attractive, but there are good reasons not to practice philosophy in this way. All people, no matter how skilled in philosophy they are, may make mistakes when arguing to a conclusion from some premises. And, not only may they make a mistake, they may make one that they are unable to detect even upon careful review (for example, omission of some possibility). Now in a short philosophical piece this isn’t really an issue; although I am aware that I may have made an error that I cannot detect I judge that the possibility is small enough to justify some confidence in my work. But a philosophical argument from first principles is not like such a short argument. Instead of going fairly straightforwardly from premises to the conclusion there must be a large number of intermediate steps, in which more and more complex principles are developed, until finally the desired philosophy is built up. (This is why the philosophers who develop their philosophy from first principles tend to write such large books, or so many books.) But in such a situation the probability of an undetectable error becomes much greater, as there are many more chances to make such an error, and so the probability that the entire work is error free becomes very small. But, since the entire work hangs together as one large argument, any error has the potential to destroy the entire enterprise, or at least cast doubt upon it, which is why, upon discovering such problems, few accept such works “as is”.
An equally serious problem is that it encourages self-deception on the part of the philosopher who creates such a work. Because it all hangs together accepting that one part is flawed implies that the whole is problematic, as I pointed out above. And this is a strong motivation to ignore the flaws in your own work, or at least assume that they can be resolved without difficulty by others, otherwise you might have to set aside the project that you have devoted so much of your time to and start again. It is my guess that this happened to Kant with the categorical imperative. I assume that he realized it had problems, but, because he didn’t want to abandon his project or admit that not everything he wanted could be derived from it, convinced himself that the problems were minor and could easily be overcome. Perhaps if Kant had worked with his categorical imperative in isolation he could have come up with a more robust version, but his desire to fit all the pieces together into a larger picture prevented this possibility.
In any case it is not currently in fashion to approach philosophy in this way. Philosophy is advanced through papers, which must be relatively short (compared to The Critique of Pure Reason), and must stand on their own, for the most part. Perhaps then I simply wrote this in order to resist my own temptation to connect everything in to one big picture, which could surely end no better.