And at this point I need not conceal my opinion that no philosopher has ever yet succeeded in avoiding this self-contradictory error: that the most striking results both of Idealism and of Agnosticism are only obtained by identifying blue with the sensation of blue: that _esse_ is held to be _percipi,_ solely because _what is experienced_ is held to be identical with _the experience of it._ That Berkeley and Mill committed this error will, perhaps, be granted: that modern Idealists make it will, I hope, appear more probable later. But that my opinion is plausible, I will now offer two pieces of evidence. The first is that language offers us no means of referring to such objects as "blue" and "green" and "sweet," except by calling them sensations: it is an obvious violation of language to call them "things" or "objects" or "terms." And similarly we have no natural means of referring to such objects as "causality" or "likeness" or "identity," except by calling them "ideas" or "notions" or "conceptions." But it is hardly likely that if philosophers had clearly distinguished in the past between a sensation or idea and what I have called its object, there should have been no separate name for the latter. They have always used the same name for these two different "things" (if I may call them so): and hence there is some probability that they have supposed these "things" _not_ to be two and different, but one and the same. And, secondly, there is a very good reason why they should have supposed so, in the fact that when we refer to introspection and try to discover what the sensation of blue is, it is very easy to suppose that we have before us only a single term. The term "blue" is easy enough to distinguish, but the other element which I have called "consciousness"--that which sensation of blue has in common with sensation of green--is extremely difficult to fix. That many people fail to distinguish it at all is sufficiently shown by the fact that there are materialists. And, in general, that which makes the sensation of blue a mental fact seems to escape us: it seems, if I may use a metaphor, to be transparent--we look through it and see nothing but the blue; we may be convinced that there _is something_ but _what_ it is no philosopher, I think, has yet clearly recognised.