In the preceding chapter, I endeavoured to shew, _first_, that nutrition is required only because waste, and a deposition of new particles, are continually going on, so that the body would speedily become exhausted if its constituent materials were not renewed; _secondly_, that the sense of appetite is given to animals for the express purpose of warning them when a fresh supply of aliment is needed--as, without some such monitor, they would be apt to neglect the demands of nature; and _thirdly_, that vegetables have no corresponding sensation, simply because, from their being at all times in communication with the soil, their nutrition goes on continuously in proportion as it is necessary, and without requiring any prompter to put it in action at particular times. If these principles be correct, it follows that, in the healthy state (and let the reader be once for all made aware that in the following pages the state of health is always implied, except where it is otherwise plainly expressed), the dictates of appetite will not be every day the same, but will vary according to the mode of life and wants of the system, and, when fairly consulted, will be sufficient to direct us both at what time and in what quantity we ought to take in either solid or liquid sustenance. But to make this perfectly evident, a few general observations may be required. It is needless to waste words in attempting to describe what hunger and thirst are: every one has felt them, and no one could understand them without such experience, any more than sweetness or sourness could be understood without tasting sweet or sour objects. Their end is manifestly to proclaim that farther nourishment is required for the support of the system; and our first business is, therefore, to explain their nature and seat, in so far at least as a knowledge of these may be conducive to our welfare.