Emerson says that the frivolous make themselves merry with this theory; and it must be admitted that at first it does not seem quite satisfactory to be told that the world in which we live is nothing more nor less than a cerebral phenomenon, which man carries with him to the tomb, and which, in the absence of a perceiver, would not exist at all. To arrive, however, at a clear understanding of the purely phenomenal existence of the exterior world, it will suffice to represent to one's self the world as it was when entirely uninhabited. At that time it was necessarily without perception. Later, there sprang up a great quantity of plants, upon which the different forces of light, air, humidity, and electricity acted according to their nature. If, now, it be remembered how impressionable plants are to these agents, and how thought leads by degrees to sensation and thence to perception, immediately then the world appears representing itself in time and space. Or, reverse the argument and imagine that the dream of the poet is realized, that nations have disappeared, and that every living thing has ceased to be, while beneath the sun's unchanging stare, and enveloped in the sky's bland, pervasive blue, the earth with her continents and archipelagoes continues to revolve in space. Under such circumstances it would naturally seem as though the universe subsisted still. But if the question is examined more closely, it will perhaps be admitted that these things remain as they are only on condition of being seen and felt. For supposing one spectator present, but of a different mental organization from our own, then the entire scene is changed; suppress him, and the whole spectacle tumbles into chaos. This doctrine, as it will be readily understood, does not in any sense deny the reality of the world in the ordinary acceptation of the term; it maintains merely that every object is conditioned by its subject; or, to explain the theory less technically, it will be sufficient to reflect that for the world, or for anything else, to be an object, there must be some one as subject to think it; for instance, the dreamless sleep proves that the earth exists only to the thinking mind, and should all Nature be rocked in an eternal slumber, there could then be no question of an exterior world. If it be asked in what this perception consists, which represents the exterior world, we find that it is limited to three fundamental concepts, that of time, space, and their concomitant causality; but inasmuch as time and space are the receptacle of every phenomenon, once their ideality is established, the ideality of the world is proven at the same moment, and with it the truth of the formula, "The world is my idea."