“The life of a thinking man will probably be divided into two parts -- the first in which he desires to exterminate modern thinkers, and the second in which he desires to watch them exterminating each other. ... Suppose, for instance, there is an old story and a new skeptic who is skeptical of the story. We have only to wait a little while for a yet newer skeptic who is skeptical of the skeptic. He will probably find the old notion actually a help in his new notion. This process is an abstract truth applying to anything, apart from agreement or disagreement.”

And vice/versa where the modern thinkers desire the same for the new skeptic. The old notion that helps is nothing more than a return to the truth as the new notion is man's attempt to cover up the old notion which was closer to the truth to begin with. Agreement is either based on lies or based on truth. Disagreement can only be a lie vs. the truth.

sometimes

This is the notion of the meme before the word meme was coined. Successful ideas spread almost in a Darwinian mode similar to the spread of successful genes, i.e. the survival of the fittest idea. It doesn't mean that any particular idea is necessarily the ultimate TRUTH, but that it is believed in by a large number of people. The most successful idea will have enthusiastic believers who want to spread that new truth.