What he appears to have studied with particular attention is the natural history of diseases, that is to say, their tendencies to a favorable or fatal issue; and without this knowledge, what can all medical practice be but blind empiricism?--a haphazard experiment, which perchance may turn out either to cure or to kill the patient? In a word, let me take this opportunity of saying, that the physician who cannot inform his patient what would be the probable issue of his complaint, if allowed to follow its natural course, is not qualified to prescribe any rational plan of treatment for its cure. One of the most distinguishing characteristics, then, of the Hippocratic system of medicine, is the importance attached in it to _prognosis_, under which was comprehended a complete acquaintance with the previous and present condition of the patient, and the tendency of the disease. To the overstrained system of Diagnosis practised in the school of Cnidos, agreeably to which diseases were divided and subdivided arbitrarily into endless varieties, Hippocrates was decidedly opposed; his own strong sense and high intellectual cultivation having, no doubt, led him to the discovery, that to accidental varieties of diseased action there is no limit, and that what is indefinite cannot be reduced to science.[43] Nothing strikes one as a stronger proof of his nobility of soul, when we take into account the early period in human cultivation at which he lived, and his descent from a priestly order, than the contempt which he everywhere expresses for ostentatious charlatanry, and his perfect freedom from all popular superstition.[44] Of amulets and complicated machines to impose on the credulity of the ignorant multitude, there is no mention in any part of his works. All diseases he traces to natural causes, and counts it impiety to maintain that any one more than another is an infliction from the Divinity. How strikingly the Hippocratic system differs from that of all other nations in their infantine state must be well known to every person who is well acquainted with the early history of medicine.[45] His theory of medicine was further based on the physical philosophy of the ancients, more especially on the doctrines then held regarding the elements of things, and the belief in the existence of a spiritual essence diffused through the whole works of creation, which was regarded as the agent that presides over the acts of generation, and which constantly strives to preserve all things in their natural state, and to restore them when they are preternaturally deranged. This is the principle which he called Nature, and which he held to be a _vis medicatrix_. “Nature,” says he, or at least one of his immediate followers says, “is the physician of diseases.”[46] His physical opinions are so important, that I have resolved to devote an entire section to an exposition of the ancient doctrines on this head. (See Sect. III.)