Why Was "Custom a Second Nature" in Early Modern Medicine?

"Custom is a second nature" is a saying that circulated long before the early modern period and in many different cultural settings. But the maxim had special salience, reference, and force in dietetic medicine from the late medieval period through the eighteenth century. What did that saying mean in the early modern medical setting? What presumptions about the body, about habitual ways of life, and about the authority of medical knowledge were inscribed within it? And what was the historical career of the saying as views of the body, its transactions with the environment, and the hereditary process changed through the nineteenth and twentieth centuries?