25. That sensations are something more than mere phenomena of our soul, that they are effects of a cause distinct from ourselves, is seen by comparing them with each other. We refer some to an external object; others we do not: these two orders of phenomena present very different characters. I now have within me the representation of the country where I was born and spent my earliest years. I see clearly a vast plain with its fields and prairies, its little hills, now forming only isolated hillocks, now stretching in various directions, sinking to the level of the plain, or gradually rising until incorporated with the mountains, the lofty chain of which surrounds all the plain, and makes it a great amphitheatre, with no outlet except on the south, and here and there a chasm, seemingly torn in the mighty wall reared by nature. All this is very perfectly represented within me, although more than a hundred leagues distant, and this whenever, and as long as I choose. The same spectacle may, perhaps, be offered to me without the concurrence of my will, but I am always free to distract myself from it; I may drop the curtain upon this scene, or raise it anew at my pleasure. What happens in this case is confirmed by many others; and thus I internally experience a series of phenomena representative of external objects, but am under no necessity to submit to them, for I can abandon or resume them by simple acts of my free will. But, at the same time, I feel within myself another class of phenomena which are not dependent upon my will, and which I cannot abandon and resume at pleasure; they are subject to certain conditions which I cannot dispense with under pain of not attaining my purpose. I now experience that a painting is represented to me; or, in ordinary language, I see a painting before me. Let us suppose this to be a purely internal phenomenon, and observe the conditions of its existence, abstracting, however, all external reality, that of my own body included, and that, also, of the organs whereby the sensation is, or seems to be, transmitted to me. Now I experience the sensation; now I do not. What has intervened? The sensation of a motion that has produced another sensation of sight, and has destroyed the first; or, passing from ideal to real language, I have placed my hand between my eyes and the object. But why can I not during the last, reproduce the first sensation? We see clearly that if external objects do exist, and my sensations are produced by them, my sensations must be subject to the conditions which they impose upon them; but if they are only internal phenomena, there is no way of explaining them. This is only the more incomprehensible as we do not find in the sensations, which we consider as mere phenomena with no immediate relation to an external object, a close dependence of some upon others, but rather, on the contrary, great discordance.