But he went on to argue that sense-data were the only
things of whose existence our perceptions could assure us; and that
to be known is to be 'in' a mind, and therefore to be mental. Hence he
concluded that nothing can ever be known except what is in some mind,
and that whatever is known without being in my mind must be in some
other mind.

In order to understand his argument, it is necessary to understand his
use of the word 'idea'. He gives the name 'idea' to anything which
is _immediately_ known, as, for example, sense-data are known. Thus a
particular colour which we see is an idea; so is a voice which we hear,
and so on. But the term is not wholly confined to sense-data. There will
also be things remembered or imagined, for with such things also we have
immediate acquaintance at the moment of remembering or imagining. All
such immediate data he calls 'ideas'.

He then proceeds to consider common objects, such as a tree, for
instance. He shows that all we know immediately when we 'perceive' the
tree consists of ideas in his sense of the word, and he argues that
there is not the slightest ground for supposing that there is anything
real about the tree except what is perceived. Its being, he says,
consists in being perceived: in the Latin of the schoolmen its '_esse_'
is '_percipi_'. He fully admits that the tree must continue to exist
even when we shut our eyes or when no human being is near it. But this
continued existence, he says, is due to the fact that God continues to
perceive it; the 'real' tree, which corresponds to what we called the
physical object, consists of ideas in the mind of God, ideas more or
less like those we have when we see the tree, but differing in the fact
that they are permanent in God's mind so long as the tree continues
to exist. All our perceptions, according to him, consist in a
partial participation in God's perceptions, and it is because of this
participation that different people see more or less the same tree. Thus
apart from minds and their ideas there is nothing in the world, nor is
it possible that anything else should ever be known, since whatever is
known is necessarily an idea.

There are in this argument a good many fallacies which have been
important in the history of philosophy, and which it will be as well to
bring to light. In the first place, there is a confusion engendered by
the use of the word 'idea'. We think of an idea as essentially something
in somebody's mind,