We can define them, in strictness, only by writing down a complete list of what we have called their characters. _Every sensation shows itself to us under various aspects_, or, as we are accustomed to say, possesses a number of =attributes=. We have been dealing, so far, with the qualitative aspect of sensations. This may itself be single; the quality of lights is just their lightness or darkness; or it may be manifold; the quality of colours can be properly described only if we take account of hue, tint, and chroma; that of tones only if we take account of pitch, volume, and tonality, perhaps also of vocality. =Quality= is the natural thing to start out from, because it is what interests us most in everyday life, and has therefore been named; so that, when we speak of sensations, we speak of them by their qualities. There are, however, several other attributes; sensations possess =intensity=, and =vividness=, and =duration=, and some of them possess =extension=. We shall discuss these aspects later on. _Does it seem strange, now, that an elementary hit of experience should turn so many sides to the observer?_ Think then of chemistry, and of the chemical elements. Sodium is a chemical element; but it has many aspects or properties; physically regarded, it is soft, it is fusible, it volatilises at high temperatures; chemically, it combines with oxygen, it decomposes water, it is univalent, it has a low atomic weight, it is electropositive, and so forth. Sodium cannot be reduced, chemically, to anything simpler than itself, but it is nevertheless many-sided. The same thing is true of sensations. So a complete list of the aspects or attributes of sensation is as near as we can come to a definition. But since that sort of statement is clumsy; since we cannot make it complete till we have observed the sensations under all their possible aspects; and since we know that mental processes are correlated with processes in the nervous system; we may adopt another plan, and _define sensation by reference to the special bodily organ with which it is connected_. Sensations are then elementary mental processes that come to us by way of skin, muscle, ear, and the rest of the sense-organs. § 16. =The Intensity of Sensation.=—A sensation may remain the same in quality, and yet vary in strength or =intensity=. A pressure may be the pressure of an ounce or of half-a-pound; it is always pressure, the same quality, but its intensity differs. The tone you get by blowing across the mouth of a bottle may be loud or faint, though it is still the same pitch, the same tone. The weight you carry may strain the arm very little or a great deal; the sensation of strain from the tendons is the same in both cases, but its intensity is different.