You know what an ism is – somebody else’s belief system. Of course, there are successful isms and isms that are less so. Shakerism was one of the less successful ones because they didn’t believe in having children (Darwin would chuckle at that one). So, all we have left of the Shakers is some charming furniture and a simple house design.
In order for an ism to be successful, it needs two things.
First, it needs to be self-consistent. It doesn’t need to hook up with reality in any grounded sense; it just needs not to get tangled up in its own tenets. Almost any ism that last longer than a decade has this. Over the long run, a good ism will polish its dogma to eliminate any glaring discrepancies. The long-running religions have all done this. Look back at the history of Christianity, for instance, and you will see a gradual change in doctrine that lets it keep going.
The second thing it needs is an escape hatch. This is a way of explaining away those troublesome times when the ism runs afoul of reality. Spiritualists, for instance, explain that their magic works only in the presence of believers. So, any attempt to empirically test spiritual goings-on is bound to fail, because the attitude of the tester is (has to be) skeptical. Even communism, the most vicious, soul-destroying ism in history, had an escape hatch. When the repeated failures of communism were pointed out, its defenders simply said that communism couldn’t really work until it was the only system in the world. Of course, communism really did work, at least for Fidel Castro, who retired recently quite a rich man.
The escape hatch for Liberalism is Conservatism; the escape hatch for Conservatism is Liberalism. They need each other.