There is no scientific evidence whatsoever of the existence of such a third entity, "X," but all our deductions have been by analogy, which proves nothing--that is, by speculation, dreaming, and unavoidably so--since in these conceptions we are close to the border line of the human mind where logical reasoning loses itself in the fog of contradiction. But at the same time there is no evidence against the conception of an entity "X"; it is not illogical, at least no more so than all such general conceptions, no more so than, for instance, that of energy or of matter. As empirical science deals with energy and matter, and entity "X" is neither, it could not be observed by any of the methods of experimental physics or chemistry. If mind is a third entity, correlated with the entities of energy and of matter, we should expect that mental activity, or entity "X," should occur not only in the highly complex transformations of energy and of matter taking place in the brains of the highest orders of living beings, but that entity "X" should appear in all physico-chemical reactions, just as energy transformations always occur in transformations of matter, and inversely. But this seems not so, and in most of the transformations of energy and of matter entity "X" does not appear. However, we have no satisfactory means of recognizing entity "X," no methods of studying it. Therefore, it may well be that it is noticed only in those rare instances when it appears of high intensity, but in most reactions entity "X" may be so small or appear in such way as to escape observation by the means and by the methods now available. Like energy or matter, entity "X" may have many forms in which it is not recognized by us, just as for a long time the flame was not recognized as the entity energy.