But since Hunger and Thirst, Joy and Grief, Pain and Terror, are also _felt_, yet are never classed under the head of Matter, the grounds of the classification of feelings have to be expressed. Professor Bain makes the distinction between Matter and Mind to rest solely on the presence or absence of Extension: this is the decisive mark: Matter he defines as the Extended. The definition is inadequate. When I see a dog and its image reflected in a pool, or see a dog and think of another, in the three cases dog, image, and idea have Extension; but I recognize the dog as a material fact, the idea as a mental fact; and although the image of the dog has material conditions by which I am optically affected, just as the idea has material cerebral conditions, I recognize a marked difference between them and the dog, due to the different modes of apprehension. The dog is known as a persistent reality, which, when Sight is supplemented by Touch, will yield sensations of Resistance, and thus disclose its materiality. The image vanishes if I attempt to touch it; I see its outlines waver and become confused with every disturbance of the surface of the pool; the idea vanishes when another idea arises; whence I conclude that neither has material reality, because neither has the Resistance which characterizes the Not-Self. The image and the idea may be referred to material conditions, but so may pains, terrors, volitions, yet these are all without Extension, simply because they are not visual feelings. 42. Matter does not represent all feelings, but only the objective sensibles; and these are not all characterized by Extension, but only those which directly or indirectly involve optico-tactical experiences accompanied by muscular experiences. Matter is primarily the Visible and Resistant; and secondarily, whatever can be imagined as such; so that ether, molecules, and atoms, although neither visible nor tangible, are ranged under the head of Matter. Color is a feeling as Sound and Scent are feelings, and although material conditions are equally presupposed in all three, yet Color alone has Extension, and because it can be imaged it has a more objective character than the others, which having no lines and surfaces, want the optical conditions for the formation of images, and are less definitely connected with tactical and muscular experiences. Nevertheless, since Sound and Scent are obviously associated with objects seen and touched, they have a degree of materiality never assigned to such feelings as Hunger and Thirst, Pleasure, Terror, and Hope.