Some philosophers are very fond of asserting that a colour cannot exist except when it is perceived; and it might possibly be thought that when I suggest that colours do really exist, I am suggesting that they do exist when they are not perceived. I wish, therefore, briefly to point out that the question whether anything does exist, when it is not perceived, is one which I have not argued and shall not attempt to argue in this paper. I have, indeed, tried to show that since "exists" does not _mean_ "is perceived," it is, at least, conceivable that things should exist, when they are not perceived. But I have admitted that it is quite possible none _do_ so: it _may_ be the case that whenever a thing exists, it is _also_ at the same time perceived, for anything that I have said or shall say to the contrary. I think, indeed, that, if such things as colours _do_ exist, my observation of their behaviour will justify me in concluding that they also exist when I myself am, at least, not aware of perceiving them: but since I have not attempted to determine what kinds of observation are sufficient to justify a generalisation, I do not pretend to say whether this is so or not: and still less do I pretend to say whether, _if_ they exist when _I_ do not perceive them, we are justified in supposing that someone else must be perceiving them. The question whether anything exists, when it is not perceived, and, if so, what things, seems to me to be one which can only be settled by observation; and thus, I conceive, observation might justify us in concluding that certain kinds of things--pains, for example, do _not_ exist, when they are not perceived and that other kinds of things--colours, for example, _do_ exist, when they are not perceived. The only way, in which, so far as I am aware, the theory I am advocating does conflict with ordinary Idealistic conclusions, is that it does suggest that things, which are _not_ "spiritual," do _sometimes_ exist, as really and as truly, as things which are.