and thus when we are told that a tree consists
entirely of ideas, it is natural to suppose that, if so, the tree
must be entirely in minds. But the notion of being 'in' the mind is
ambiguous. We speak of bearing a person in mind, not meaning that the
person is in our minds, but that a thought of him is in our minds. When
a man says that some business he had to arrange went clean out of his
mind, he does not mean to imply that the business itself was ever in his
mind, but only that a thought of the business was formerly in his mind,
but afterwards ceased to be in his mind. And so when Berkeley says that
the tree must be in our minds if we can know it, all that he really has
a right to say is that a thought of the tree must be in our minds. To
argue that the tree itself must be in our minds is like arguing that a
person whom we bear in mind is himself in our minds. This confusion
may seem too gross to have been really committed by any competent
philosopher, but various attendant circumstances rendered it possible.
In order to see how it was possible, we must go more deeply into the
question as to the nature of ideas.

Before taking up the general question of the nature of ideas, we must
disentangle two entirely separate questions which arise concerning
sense-data and physical objects. We saw that, for various reasons of
detail, Berkeley was right in treating the sense-data which constitute
our perception of the tree as more or less subjective, in the sense that
they depend upon us as much as upon the tree, and would not exist if the
tree were not being perceived. But this is an entirely different point
from the one by which Berkeley seeks to prove that whatever can be
immediately known must be in a mind. For this purpose arguments of
detail as to the dependence of sense-data upon us are useless. It is
necessary to prove, generally, that by being known, things are shown to
be mental. This is what Berkeley believes himself to have done. It
is this question, and not our previous question as to the difference
between sense-data and the physical object, that must now concern us.

Taking the word 'idea' in Berkeley's sense, there are two quite distinct
things to be considered whenever an idea is before the mind. There is
on the one hand the thing of which we are aware--say the colour of my
table--and on the other hand the actual awareness itself, the mental act
of apprehending the thing. The mental act is undoubtedly mental, b