When we ask what the word "_justified_" can mean in this connection, it is not easy to find an answer. Within the sphere of the immediately known the meaning of the word is plain enough. When I have constructed in my imagination a certain image or complex of images embodying a belief as to matter of fact, I say the mental operation is justified when I can substitute for the idea the percept which it is supposed to represent, or can know indirectly that this might be done according to known laws of the appearance and disappearance of percepts. Thus I perceive the outside of a tree-trunk and form an idea of what lies under the bark. I have reason to know that by stripping off the bark I can substitute for the image I have formed the corresponding percept. And if I see at a distance a similar tree growing upon an inaccessible cliff, and form an image of what lies under its bark, I may still regard this as justified by the possibility of referring to cases in which a similar image, arising out of a similar experience, has been found to be justified. It is a legitimate inference that, if circumstances were somewhat different, the proper percept might take the place of this image too. It is evident, however, that the word "justified" cannot be used in this or any analogous sense in speaking of the relation not of an image to a percept, but of an image or a percept to a something that, by hypothesis, cannot itself enter into experience at all. What then can the word mean? It at first interests us to know that "some Snarks are Boojums," but our interest lapses when we discover that we have absolutely no mark by which we may know a Boojum from anything else.