"Berkeley, as we have seen, thinks that there are logical reasons proving that only minds and mental events can exist. This view, on other grounds, is also held by Hegel and his followers. I believe this to be a complete mistake. Such a statement as “there was a time before life existed on this planet,” whether true or false, cannot be condemned on grounds of logic, any more than “there are multiplication sums which no one will have ever worked out.” To be observed, or to be a percept, is merely to have effects of certain kinds, and there is no logical reason why all events should have effects of these kinds."