From The Theory of Moral Sentiments:
Amidst the turbulence and disorder of faction, a certain spirit of system is apt to mix itself with that public spirit which is founded upon the love of humanity, upon a real fellow-feeling with the inconveniencies and distresses to which some of our fellow-citizens may be exposed. This spirit of system commonly takes the direction of that more gentle public spirit; always animates it, and often inflames it even to the madness of fanaticism. The leaders of the discontented party seldom fail to hold out some plausible plan of reformation which, they pretend, will not only remove the inconveniencies and relieve the distresses immediately complained of, but will prevent, in all time coming, any return of the like inconveniencies and distresses. They often propose, upon this account, to new-model the constitution, and to alter, in some of its most essential parts, that system of government under which the subjects of a great empire have enjoyed, perhaps, peace, security, and even glory, during the course of several centuries together. The great body of the party are commonly intoxicated with the imaginary beauty of this ideal system, of which they have no experience, but which has been represented to them in all the most dazzling colours in which the eloquence of their leaders could paint it. Those leaders themselves, though they originally may have meant nothing but their own aggrandisement, become many of them in time the dupes of their own sophistry, and are as eager for this great reformation as the weakest and foolishest of their followers. Even though the leaders should have preserved their own heads, as indeed they commonly do, free from this fanaticism, yet they dare not always disappoint the expectation of their followers; but are often obliged, though contrary to their principle and their conscience, to act as if they were under the common delusion. The violence of the party, refusing all palliatives, all temperaments, all reasonable accommodations, by requiring too much frequently obtains nothing; and those inconveniencies and distresses which, with a little moderation, might in a great measure have been removed and relieved, are left altogether without the hope of a remedy.
The key part about self-deception:
Those leaders themselves, though they originally may have meant nothing but their own aggrandisement, become many of them in time the dupes of their own sophistry, and are as eager for this great reformation as the weakest and foolishest of their followers. Even though the leaders should have preserved their own heads, as indeed they commonly do, free from this fanaticism, yet they dare not always disappoint the expectation of their followers; but are often obliged, though contrary to their principle and their conscience, to act as if they were under the common delusion.
