How shall we know if we are in our best condition?
First, we shall not be thinking about it at all. We shall not mind about the quality of our food very much. Life will hold other pleasures for us.
Mere motion, action, work, that is, use of muscular power, brings a delightful sense of life and force. The healthy workman goes to his day's work with vigor in his step, the school boy to his desk with eagerness.
If we find ourselves sluggish and tired in the morning it is because something is wrong. The standard of good health is for all alike the consciousness of power. We ask, How much power of work is there in the food we eat? - how much food do we need for a day's work? We call this power Energy, and we reckon the force in Calories, that is in the mechanical equivalent of heat. This is the starting point of all our modern work in dietetics.
The modern standard, then, of good health is energy, power to do work; and by work we mean thinking, inventing, painting, writing, just as much as swinging a sledge hammer.