Though sometimes amusing, it is always disturbing when intelligent people seriously talk nonsense. This is true even of the dead; we are always surprised at the credulity of our forebears. Sometimes it shocks us; they often held views that seem wicked as well as false, and they sometimes acted on them. It ought to be merely a truism that bad ideas can be as effective and influential as good, but obviously it is not, since we are so surprised by this. The hardest things to understand about much of the past are its errors and delusions. We are shut off from understanding them not only by the difficulties of research and by insensibility, for these are only general and preliminary obstacles to any discovery of the past, but also by the particular, anachronistic incredulity which we bring to anything which does not rest on our own intellectual assumptions.