Cusanus was an extremely practical man, and was constantly looking for and devising methods of applying practical principles of science to ordinary life. As we shall see in discussing his suggestion for the estimation of the pulse rate later on, he made many other similar suggestions for diagnostic purposes in medicine, and set forth other applications of mathematics and mechanics to his generation. Many of Cusanus' books have curiously modern names. He wrote, for instance, a series of mathematical treatises, in Latin of course, on "Geometric Transmutations," on "Arithmetical Complements," on "Mathematical Complements," on "Mathematical Perfection," and on "The Correction of the Calendar." In his time the calendar was in error by more than nine days, and Cusanus was one of those who aroused sufficient interest in the subject, so that in the next century the correction was actually made by the great Jesuit mathematician, Father Clavius. Perhaps the work of Cusanus that is best known is that "On Learned Ignorance--De Docta Ignorantia," in which the Cardinal points out how many things that educated people think they know are entirely wrong. It reminds one very much of Josh Billings's remark that it is not so much the ignorance of mankind that makes them ridiculous, as the knowing so many things that ain't so. It is from this work that the astronomical quotations which we have made are taken. The book that is of special interest to physicians is his dialogue "On Static Experiments," which he wrote in 1450, and which contains the following passages: "Since the weight of the blood and the urine of a healthy and of a diseased man, of a young man and an old man, of a German and an African, is different for each individual, why would it not be a great benefit to the physician to have all of these various differences classified? For I think that a physician would make a truer judgment from the weight of the urine viewed in connection with its color than he could make from its color alone, which might be fallacious. So, also, weight might be used as a means of identifying the roots, the stems, the leaves, the fruits, the seeds, and the juice of plants if the various weights of all the plants were properly noted, together with their variety, according to locality. In this way the physician would appreciate their nature better by means of their weight than if he judged them by their taste alone. He might know, then, from a comparison of the weights of the plants and their various parts when compared with the weight of the blood and the urine, how to make an application and a dosage of drugs from the concordances and differences of the medicaments, and even might be able to make an excellent prognosis in the same way. Thus, from static experiments, he would approach by a more precise knowledge to every kind of information.