Whoever wishes to investigate medicine properly, should proceed thus: in the first place to consider the seasons of the year, and what effects each of them produces; for they are not at all alike, but differ much from themselves in regard to their changes. Then the winds, the hot and the cold, especially such as are common to all countries, and then such as are peculiar to each locality. We must also consider the qualities of the waters; for as they differ from one another in taste and weight, so also do they differ much in their qualities. In the same manner, when one comes into a city to which he is a stranger, he ought to consider its situation, how it lies as to the winds and the risings of the sun; for its influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. These things one ought to consider most attentively, and concerning the waters which the inhabitants use, whether they be marshy and soft, or hard and running from elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered; and whether it lies in a hollow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and of indolence, or are fond of exercise and labor.

From these things, one must proceed to investigate the diseases most common to the city and the nature of each disease; for in a city it is impossible that the diseases should not be such as the seasons, the winds, the waters, and the soil determine. If one knows all these things well, he will be able to recognize the most frequent diseases in each season, and the most dangerous ones, and generally speaking, he will not be at a loss for treatment. For one then knows the constitutions of the year and can foresee the diseases that must happen either in summer or in winter. If one finds that the season is going to be such and such, he can prepare himself for the treatment. But if these things are ignored, one can foresee nothing and will often be deceived. Thus, one must carefully study the nature of the seasons, and above all the sudden changes, which are the most dangerous. For what is most important in medicine is to know the changes of the seasons and the diseases that result from them. Then, in each region, one must examine the waters and the winds, and the way the inhabitants feed and live. For diseases differ according to countries, and one must adapt the treatment to these differences. Thus, in countries where the waters are hard, cold, saltish, or of another nature, the diseases differ from those that prevail in countries where the waters are sweet, warm, or of good quality. Likewise, the north and south winds produce different effects on the body. Some bring acute diseases, others chronic ones. One must therefore observe all this carefully in order to practice medicine well.

I will now set forth what I have learned on these subjects by observation and experience. I shall begin with the seasons, then I shall speak of the winds, then of the waters, and finally of the soil. For these are the principal elements that determine health and diseases. And I shall show how each thing acts upon the human body, and how one can prevent diseases by knowing these influences. Thus, he who studies medicine according to these principles will never be taken unawares, and he will be able to treat the sick with more success. For medicine does not consist only in giving remedies, but in knowing the causes of diseases and avoiding them. Therefore, I advise all those who wish to become good physicians to begin with the study of air, waters, and places.