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I’ve always been intrigued by (and placed a fair amount of stock in) the Keirsey/Myers/Briggs temperament classification system, yet I hadn’t ever read this about INTPs before:

A major concern for INTPs is the haunting sense of impending failure. They spend considerable time second-guessing themselves. The open-endedness (from Perceiving) conjoined with the need for competence (NT) is expressed in a sense that one’s conclusion may well be met by an equally plausible alternative solution, and that, after all, one may very well have overlooked some critical bit of data. An INTP arguing a point may very well be trying to convince himself as much as his opposition. In this way INTPs are markedly different from INTJs, who are much more confident in their competence and willing to act on their convictions.

That explains a lot, frankly.