We've largely lost the ancient art of dialectic as taught in Aristotle's Topics and the like. We tend to equate it with formal logic, the art of "judgment," but it was traditionally understood primarily as an art of invention and only of judgment secondarily. Boethius is partly responsible for this change, but it is largely the fault of logicians in the late medieval period. This fact alone was not a big problem, since the rhetoricians still taught the art of invention (until they stopped doing it, too). The main issue with the new dialectic was that it incorporated a whole slew of new "rules" that twisted language into barbaric forms, disparaged common sense, and promoted speculation about vanities. Insofar as the humanists attack these aspects of medieval dialectic, I sympathize with them. In his introduction to English Literature in the Sixteenth Century, C.S. Lewis comes down pretty hard on the humanists, claiming that they merely "jeer and do not refute" when attacking the scholastics. But works like this one demonstrate that some of their attacks had real substance, and at least men like Valla proposed their own philosophical systems. There are many things to be thankful for concerning medieval dialectic and philosophy, and the best humanists acknowledged this. But it certainly became corrupted, and we can thank the humanists for exposing this.