What is the difference between hunger and appetite?

In the matter of how much to eat, you will also see that there could be no other guide so natural, so safe, and so reliable as the one I have prescribed—to stop eating on the instant you feel that your hunger begins to abate. The sub-conscious mind may be trusted with implicit reliance to inform us when food is needed; and it may be trusted as implicitly to inform us when the need has been supplied. If ALL food is eaten for hunger, and NO food is taken merely to gratify taste, you will never eat too much; and if you eat whenever you have an EARNED hunger, you will always eat enough. By reading carefully the summing up in the following chapter, you will see that the requirements for eating in a perfectly healthy way are really very few and simple.

The matter of drinking in a natural way may be dismissed here with a very few words. If you wish to be exactly and rigidly scientific, drink nothing but water; drink only when you are thirsty; drink whenever you are thirsty, and stop as soon as you feel that your thirst begins to abate. But if you are living rightly in regard to eating, it will not be necessary to practice asceticism or great self-denial in the matter of drinking. You can take an occasional cup of weak coffee without harm; you can, to a reasonable extent, follow the customs of those around you. Do not get the soda fountain habit; do not drink merely to tickle your palate with sweet liquids; be sure that you take a drink of water whenever you feel thirst. Never be too lazy, too indifferent, or too busy to get a drink of water when you feel the least thirst; if you obey this rule, you will have little inclination to take strange and unnatural drinks. Drink only to satisfy thirst; drink whenever you feel thirst; and stop drinking as soon as you feel thirst abating. That is the perfectly healthy way to supply the body with the necessary fluid material for its internal processes.