Whoever wishes to pursue medicine accurately must do this: first, consider the seasons of the year, and what each of them can do; for they are not alike, but differ greatly from each other and in their changes. Second, the winds, hot and cold, and especially the universal one, and then those peculiar to each region. Also, the qualities of the waters must be considered; for, just as they differ in taste and weight, so they differ greatly in their influence. Consequently, when one arrives in a town whose situation is not yet known, one must carefully examine the position of that town with respect to the winds and the rising of the sun; for the influence is not the same whether it is exposed to the north or the south, to the east or the west. One must also consider the waters, whether they are marshy and soft, or hard and from high, rocky places, or salty and harsh. The soil too, whether it is bare and dry, or wooded and damp; whether it is hollow and stifling, or high and cold. The lifestyle of the inhabitants, whether they are fond of drinking and eating, and given to idleness, or whether they love work and exercise, and are sober and industrious.\n\nThese are the considerations with which one must begin. Then, for each disease, one will examine the seasons, the ages, the temperaments, the habits, and everything that can modify the patient's condition. For it is on this that the judgment of the prognosis and the success of treatment depend.\n\nThus, he who possesses these well will be able, in a town, to foresee what epidemic diseases will prevail, either in summer or in winter, and also what particular affections will occur to each individual through change of regimen. For knowing the changes of the seasons, the rising and setting of the stars, and how each of these phenomena occurs, he will know in advance what is to happen for each year. Thus, with this foresight, he will know what to prescribe, and will not be deceived in his hopes, if he has rightly grasped the critical moment. But for those who do not possess this knowledge, it is impossible to judge these things soundly, and they will often be mistaken.\n\nAs for waters, one must consider those that are stagnant and putrid, and those that are fresh and flowing. Stagnant waters are heavy, thick, and cause obstructions; fresh waters are light and easy to digest. Rainwater is the best, for it is light, sweet, and subtle; but it quickly spoils. Spring waters differ according to the nature of the ground; those from rocky places are hard and cold; those from clayey soil are turbid and heavy. Also, one must examine whether the waters are salty, for they are bad for the stomach. Hot waters are soft and relaxing; cold waters are astringent and strengthening.\n\nWinds also have a great influence: the north wind constricts the body and makes it drier; the south wind softens and moistens it. The east wind is salutary; the west wind is harmful, because it is variable and laden with moisture. The seasons likewise modify the condition of the body: in spring, the body is more disposed to purgations; in summer, it is more subject to fevers; in autumn, to dysenteries; in winter, to catarrhs. Therefore, the physician must observe all these things with care, and regulate his treatment according to circumstances.\n\nMoreover, the ages: children are prone to acute diseases; young people, to fevers and inflammations; adults, to chronic diseases; the elderly, to catarrhs and paralyses. The temperaments too: the bilious are dry and hot; the phlegmatic, damp and cold; the sanguine, hot and damp; the melancholic, cold and dry. Each temperament has its own diseases. Thus, the physician who knows all this will be able to judge the present state and predict the future, and he will be esteemed as a true practitioner.\n\nTherefore, the physician must be attentive to all these signs, and neglect no observation. For the nature of diseases is manifested through the seasons, waters, winds, and manner of life. He who knows how to read these signs will have a more certain knowledge of the medical art. And one must not believe that medicine is a conjectural science; it is founded on certain principles, deduced from experience and reason. Thus, whoever wishes to practice medicine successfully must study these things from the beginning.