I cannot say that, from my own experience, I have found that the persons most remarkable for sudden and violent changes of principle have been cast in the softest or most susceptible mould. All their notions have been exclusive, bigoted, and intolerant. Their want of consistency and moderation has been in exact proportion to their want of candour and comprehensiveness of mind. Instead of being the creatures of sympathy, open to conviction, unwilling to give offence by the smallest difference of sentiment, they have (for the most part) been made up of mere antipathies— a very repulsive sort of personages— at odds with themselves, and with everybody else. The slenderness of their pretensions to philosophical inquiry has been accompanied with the most presumptuous dogmatism. They have been persons of that narrowness of view and headstrong self-sufficiency of purpose, that they could see only one side of a question at a time, and whichever they pleased. There is a story somewhere in Don Quixote, of two champions coming to a shield hung up against a tree with an inscription written on each side of it. Each of them maintained, that the words were what was written on the side next him, and never dreamt, till the fray was over, that they might be different on the opposite side of the shield. It would have been a little more extraordinary if the combatants had changed sides in the heat of the scuffle, and stoutly denied that there were any such words on the opposite side as they had before been bent on sacrificing their lives to prove were the only ones it contained. Yet such is the very situation of some of our modern polemics. They have been of all sides of the question, and yet they cannot conceive how an honest man can be of any but one — that which they hold at present. It seems that they are afraid to look their old opinions in the face, lest they should lie fascinated by them once more. They banish all doubts of their own sincerity by inveighing against the motives of their antagonists. There is no salvation out of the pale of their strange inconsistency. They reduce common sense and probity to the straitest possible limits — the breasts of themselves and their patrons. They are like people out at sea on a very narrow plank, who try to push everybody else off. Is it that they have so little faith in the course to which they have become such staunch converts, as to suppose that, should they allow a grain of sense to their old allies and new antagonists, they will have more than they? Is it that they have so little consciousness of their own disinterestedness, that they feel, if they allow a particle of honesty to those who now differ with them, they will have more than they? Those opinions must needs be of a very fragile texture which will not stand the shock of the least acknowledged opposition, and which lay claim to respectability by stigmatising all who do not hold them as 'sots, and knaves, and cowards.' There is a want of well-balanced feeling in every such instance of extravagant versatility; a something crude, unripe, and harsh, that does not bit a judicious palate, but sets the teeth on edge to think of. 'I had rather hear my mother's cat mew, or a wheel grate on the axletree, than one of these same metre-ballad-mongers' chaunt his incondite, retrograde lays, without rhyme and without reason.
The principles and professions change: the man remains the same. There is the same spirit at the bottom of all this pragmatical fickleness and virulence, whether it runs into one extreme or another: to wit, a confinement of view, a jealousy of others, an impatience of contradiction, a want of liberality in construing the motives of others, either from monkish pedantry, or a conceited overweening reference of everything to our own fancies and feelings."
-- ("On Consistency of Opinion" by William Hazlitt)