And seeing that, like other machines, it derives the whole of its energy from its fuel, the subject of foods—their properties, uses, and methods of preparation—has been gone into with unusual care. An adequate supply of clean-burning food-fuel for the human engine is so absolutely fundamental both for health and for efficiency—we are so literally what we have eaten—that to be well fed is in very fact two-thirds of the battle of life from a physiological point of view. The whole discussion is in accord with the aim, kept in view throughout the book, of making its suggestion and advice positive instead of negative, pointing out that, in the language of the old swordsman, "attack is the best defense."