sponding to our sensations, and by which our sensations were induced. This was a matter on which probably no great attention had been bestowed; and therefore we cannot say very exactly what the previous doctrine in regard to sensation and perception may have been; but we are safe in affirming that it had been loosely assumed that there were, as I have said, certain qualities or agents corresponding to our sensations, and by which our sensations were induced. That, I think, we may say, was the general opinion, as it is indeed the vulgar, if not the philosophic, opinion to this day. When we feel the sensation of heat, we suppose there is some corresponding quality in the fire, or whatever the agent may be which induces it. When we see coloured objects we think that the colours are in the objects themselves, or, at any rate, that there is some quality in the object which causes our sensations of colour. When we have the sensation in our mouths of sweet or of bitter, we suppose that these different tastes are excited by different qualities in the objects. The Atomic theory corrected or modified this opinion, and this correction followed as a consequence of the Atomic doctrine in regard to the constitution of material things. If the atoms, the ultimate constituents of all things, are identical in point of quality, and differ only in size, shape, position, and arrangement, it follows that there can be nothing in real nature corresponding to what we call heat or cold, or sweet or bitter, or colour. These are merely sensations in us;
