I do not propose, at present, to decide between these four points of view. The decision, if
possible at all, can only be made by an elaborate investigation of non-demonstrative inference
and the theory of probability. What I do propose to do is to point out certain logical errors
which have been committed by those who have discussed these questions.
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.
There is, however, another kind of argument, which, while it does not establish idealism as a
metaphysic, does, if valid, establish it as a practical policy. It is said that a proposition which is
unverifiable has no meaning; that verification depends upon percepts; and that, therefore, a
proposition about anything except actual or possible percepts is meaningless. I think that this
view, strictly interpreted, would confine us to the first of the above four theories, and would
forbid us to speak about anything that we have not ourselves explicitly noticed. If so, it is a
view that no one can hold in practice, which is a defect in a theory that is advocated on practical
grounds. The whole question of verification, and its connection with knowledge, is difficult and