If the different customs and usages of the Ancients in connection with their careful attention to cleanliness are examined more minutely, they are found to be divisible into two classes, according as (1.) their object was to prevent uncleanliness, or (2.) to banish it, when once admitted. All measures connected with sanitary police supervision, the enforcement of which in modern civilized States leads to such endless difficulties, were almost entirely in the hands of the Priests, to whom the People were accustomed to accord an unquestioning obedience. It was an easy matter therefore to prevent any injurious contamination from extending over a wide area; it sufficed simply to declare unclean whatever might prove injurious to health to ensure its being avoided in practice,—and in the majority of instances with the most scrupulous care. This is a factor in the problem that appears never to have been properly appreciated by our Historical Pathologists; otherwise they must long ago have abandoned many prejudices regarding the knowledge possessed by the Ancients as to contagious matter. For how _could_ practical observations be collected on infection and the liability to infection, when every possible chance of infection was carefully and generally avoided? Most of the Peoples of Antiquity considered contact with a dead body a pollution, more than this, they thought even the neighbourhood of a corpse to have the same effect. They hung up notices to warn the passers-by, and placed vessels of water (ἀδάνιον, ὄστρακον, γάστρα—water-stoup, earthen vessel, water-pot) before the house where a dead man lay, that those who came in and out might be able to purify themselves again on the spot[209]. Of course all did not go so far as the Persians, who declared every sick person unclean. Still it is a fact, and this most certainly not merely among the Jews, that all the various infectious skin-diseases that were massed together under the name of Leprosy[210], and also Gonorrhœa (Clap), made the sufferer, and also everything he touched, unclean, and caused them to be set apart where no one should come in contact with them; and this continued so long as the sickness lasted. Now does it really need any further proof that these diseases developed a perfectly well-known form of contagious matter: or is an arbitrary and imaginary theory to be adopted by preference, to the effect that injunctions of the sort owed their existence merely to the caprice of the Legislator, and were not based on any actual experience of real detriment resulting from their neglect in favour of others? At any rate it is certain that, where these laws were in force and where each individual followed them out exactly, a disease that is communicable only by close contact could not possibly be disseminated over any wide area. This could not take place under such circumstances, even though it had been engendered in its original form and continued prevalent for a long period of time.