The principle of philosophical pragmatism is that we should judge an idea by its practical effects. This is so contrary to common sense that I have always dismissed it as unworthy of further consideration. When I listened to a lecture on pragmatism by a university professor, he was apologetic about the apparent absurdity of his subject, but was unable to make it sound any more reasonable.
I gave it yet another try only because I happened to work for a company, Pragmatics, which was named after this very philosophy -- so much so that the main conference room in the headquarters building was named the Charles Sanders Peirce room. Since the founder of the company seemed to be an intelligent person, and since he was impressed by pragmatism, I figured there was probably something to it that I wasn't grasping. There is, and I think I understand it better now.
The central tenet of pragmatism is the "pragmatic maxim," in which Charles Peirce explained: "Consider what effects, that might conceivably have practical bearings,
we conceive the object of our conception to have. Then, our conception
of these effects is the whole of our conception of the object." What I think he was trying to say was simply that our understanding of an object is summed up in our interactions with that object, i.e. our practical experience. We believe that a rock will hurt if someone throws it at us, that we can scratch the rock with an item of a certain hardness, and that we can sell it for a certain amount if it is at least semiprecious. Our understanding of that rock is summed up by these practical considerations.
What else would it be? Well, if we were Aristotelians, we might believe that a rock was created for a certain purpose. That wouldn't pass the pragmatic test. Nor would an attempt to get at the essence of rock-ness. There is no point in trying to understand the true nature of a rock; the only thing we can do is understand how the rock interacts with us and the rest of the world.
To me, this sounds a lot like Kant's empirical realism. True, Kant believed that there was a fundamental nature of the rock, but he thought we could never get at it -- what he called the "thing-in-itself" (or "Ding-an-sich"). Empirically, however, he believed we could understand things based on our interactions with them.
What separates pragmatism is its ruthless emphasis on practical value, especially by people other than Charles Peirce. William James talked about "truth's cash value" and wrote that "the true is only the expedient in our way of thinking"; others said that the truth is what "works" or what "gives satisfaction." James defended his statements as being more "subtle" that they appear on the surface, but he must have realized, in that case, that he was saying something that was not straightforwardly true. It hasn't helped that contemporary "neopragmatists" like Richard Rorty have carried things even further, saying that "Truth is what your contemporaries let you get away with."
Whether any of these statements are truly representative of Peirce's original concept of pragmatism, I don't know. I am encouraged by the fact that Peirce later coined a new term, pragmaticism, to differentiate his ideas from those of his contemporaries, which suggests that he might have disagreed with them. They seem to hint at an extreme relativism which doesn't suit me and I don't think would have suited Peirce. (One problem with defining truth as "that which works" is that it implies a complete lack of morality, which really is beside the point, because pragmatism is about epistemology and, as far as I know, does not seem to have a moral teaching.)
On the other hand, I do see a value in Peirce's emphasis on the practical, as long as we distinguish it from the types of statements made by other pragmatists quoted above. He said that pragmatism was implicit in the definition of belief as "that upon which a man is prepared to act," and I think I philosophy that begins on that basis has a strong foundation. In other words, you may claim that the external world exists only in your mind, but you still take the trouble to avoid falling into a pit. You may claim that there is no independent truth, but you still argue in favour of your viewpoint. I don't know if Peirce would agree with these specific points, but he was definitely trying to create a philosophy that corresponded to how people actually did science, as opposed to the purely speculative endeavours of Hegel and his ilk.