ally is. The colour which an object seems
to have at any given moment will in general be very similar, though
not quite the same, from many different points of view; we might thus
suppose the 'real' colour to be a sort of medium colour, intermediate
between the various shades which appear from the different points of
view.

Such a theory is perhaps not capable of being definitely refuted, but
it can be shown to be groundless. To begin with, it is plain that the
colour we see depends only upon the nature of the light-waves that
strike the eye, and is therefore modified by the medium intervening
between us and the object, as well as by the manner in which light is
reflected from the object in the direction of the eye. The intervening
air alters colours unless it is perfectly clear, and any strong
reflection will alter them completely. Thus the colour we see is a
result of the ray as it reaches the eye, and not simply a property of
the object from which the ray comes. Hence, also, provided certain waves
reach the eye, we shall see a certain colour, whether the object from
which the waves start has any colour or not. Thus it is quite gratuitous
to suppose that physical objects have colours, and therefore there is no
justification for making such a supposition. Exactly similar arguments
will apply to other sense-data.

It remains to ask whether there are any general philosophical arguments
enabling us to say that, if matter is real, it must be of such and such
a nature. As explained above, very many philosophers, perhaps most, have
held that whatever is real must be in some sense mental, or at any rate
that whatever we can know anything about must be in some sense mental.
Such philosophers are called 'idealists'. Idealists tell us that what
appears as matter is really something mental; namely, either (as Leibniz
held) more or less rudimentary minds, or (as Berkeley contended) ideas
in the minds which, as we should commonly say, 'perceive' the matter.
Thus idealists deny the existence of matter as something intrinsically
different from mind, though they do not deny that our sense-data are
signs of something which exists independently of our private sensations.
In the following chapter we shall consider briefly the reasons--in my
opinion fallacious--which idealists advance in favour of their theory.

CHAPTER IV. IDEALISM

The word 'idealism' is used by different philosophers in somewhat
different senses. We shall under