to level his design: he must know the sick person's complexion, his temperament, his humours, inclinations, actions, nay, his very thoughts and imaginations; he must be assured of the external circumstances, of the nature of the place, the quality of the air and season, the situation of the planets, and their influences: he must know in the disease, the causes, prognostics, affections, and critical days; in the drugs, the weight, the power of working, the country, figure, age, and dispensation, and he must know how rightly to proportion and mix them together, to beget a just and perfect symmetry; wherein if there be the least error, if amongst so many springs there be but any one out of order, 'tis enough to destroy us. God knows with how great difficulty most of these things are to be understood: for (for example) how shall the physician find out the true sign of the disease, every disease being capable of an infinite number of indications? How many doubts and controversies have they amongst themselves upon the interpretation of urines? otherwise, whence should the continual debates we see amongst them about the knowledge of the disease proceed? how could we excuse the error they so oft fall into, of taking fox for marten? In the diseases I have had, though there were ever so little difficulty in the case, I never found three of one opinion: which I instance, because I love to introduce examples wherein I am myself concerned. A gentleman at Paris was lately cut for the stone by order of the physicians, in whose bladder, being accordingly so cut, there was found no more stone than in the palm of his hand; and in the same place a bishop, who was my particular friend, having been earnestly pressed by the majority of the physicians whom he consulted, to suffer himself to be cut, to which also, upon their word, I used my interest to persuade him, when he was dead and opened, it appeared that he had no malady but in the kidneys. They are least excusable for any error in this disease, by reason that it is in some sort palpable; and 'tis thence that I conclude surgery to be much more certain, by reason that it sees and feels what it does, and so goes less upon conjecture; whereas the physicians have no 'speculum matricis', by which to examine our brains, lungs, and liver.