When an idea goes out of fashion it is not always because it is shown to be false or discovered to neglect important considerations. It can also be discarded prematurely because another idea ascends and conflicts with it, or even because the new idea makes the old idea seem dull and played out and sucks all attention away from it. An idea can also be abandoned and forgotten because of a change in collective mood, or because the philosophical drift of a time renders an idea irrelevant, then incomprehensible, then nonsensical. Or a whole culture can decay into ideological microomniscience: all thought beyond the ideology’s horizon is self-evidently immoral, ignorant or both.