If the idealist I have imagined as making the foregoing retort were up in recent realistic literature, he might add the following argument _ad hominem_: "You, my realistic opponent, say that the doctrine of the external relation of terms expresses a ubiquitous mark of every genuine proposition or relational complex, and that this ubiquity is a strong presumption in favor of realism. Why so uneven, so partial, in your attitude toward ubiquitous relations? Is it perchance that you were so uneasy at our possession of a ubiquitous relation that gives a short cut to idealism that you felt you must also have a short cut to realism?" If I terminate the controversy at this point, it is not because I think the realist is unable to "come back." On the contrary, I stop here because I believe (for reasons that will come out shortly) that both realist and idealist, having the same primary assumption, can come back at each other indefinitely. Consequently, I wish to employ the existence of this _tu quoque_ controversy to raise the question: Under what conditions is the relation of knower to known an intelligible question? And I wish to show that it is _not_ intelligible, if the knowledge relation be ubiquitous and homogeneous. The controversy back and forth is in fact a warning of each side by the other not to depart from their _common_ premise. If the idealist begins to argue (as he constantly does) as if the relation to "mind" or to "consciousness" made some difference of a specific sort, like that between error and fact, or between sound perception and hallucination, he may be reminded that, since this relation is uniform, it substantiates and nullifies all things alike. And the realist is quite within the common premise when he points out that every special fact must be admitted for what it is specifically known to be; no idealistic doctrine can turn the edge of the fact that knowledge has evolved historically out of a state in which there was no mind, or of the fact that knowledge is even now dependent on the brain, provided that specific evidence shows these to be facts. The realist, on the other hand, must admit that, after all, the entire body of known facts, or of science, including such facts as the above, is held fast and tight in the net of relation to a mind or consciousness. In specific cases this relation may be ignored, but the exact ground for such an ignoring is precisely that the relation is not a specific fact, but a uniform relation of facts. And to call it an external relation makes no practical difference if it is universal and uniform. So the idealist might reply.