Incapable: that happens when the acceptance of an idea implies that their perception of their identity is flawed and has, logically to change in order to adapt for the new reality where the idea has its place. Denial is a protection mechanism, and it is very effective when the reality is too difficult to support as it is. Identity is so essential in our beliefs, attitudes and behaviours that most of us won't accept anything that requires it to change. Unless we accept that failure is part of our identity and that this means that our identity sometimes has to evolve. But that has to be done willingly, explicitly (in our minds).