There are habits of body suited for an active life and others for a sedentary life. The latter leaves the humours an equable and uniform course, and the body should be protected from changes in temperature; the former is constantly passing from action to rest, from heat to cold, and the body should be inured to these changes. Hence people, engaged in sedentary pursuits indoors, should always be warmly dressed, to keep their bodies as nearly as possible at the same temperature at all times and seasons. Those, however, who come and go in sun, wind, and rain, who take much exercise, and spend most of their time out of doors, should always be lightly clad, so as to get used to the changes in the air and to every degree of temperature without suffering inconvenience. I would advise both never to change their clothes with the changing seasons, and that would be the invariable habit of my pupil Emile. By this I do not mean that he should wear his winter clothes in summer like many people of sedentary habits, but that he should wear his summer clothes in winter like hard-working folk. Sir Isaac Newton always did this, and he lived to be eighty. Emile should wear little or nothing on his head all the year round. The ancient Egyptians always went bareheaded; the Persians used to wear heavy tiaras and still wear large turbans, which according to Chardin are required by their climate. I have remarked elsewhere on the difference observed by Herodotus on a battle-field between the skulls of the Persians and those of the Egyptians. Since it is desirable that the bones of the skull should grow harder and more substantial, less fragile and porous, not only to protect the brain against injuries but against colds, fever, and every influence of the air, you should therefore accustom your children to go bare-headed winter and summer, day and night. If you make them wear a night-cap to keep their hair clean and tidy, let it be thin and transparent like the nets with which the Basques cover their hair. I am aware that most mothers will be more impressed by Chardin's observations than my arguments, and will think that all climates are the climate of Persia, but I did not choose a European pupil to turn him into an Asiatic.