Sydenham's nosological method is essentially the modern one, except that it lacked the morbid anatomy part, which was first introduced into the natural history of disease by Morgagni nearly a century later. In both departments of nosology, the acute and the chronic, Sydenham contributed largely to the natural history by his own accurate observation and philosophical comparison of case with case and type with type. The "Observationes medicae" and the first "Epistola responsoria" contain evidence of a close study of the various fevers, fluxes and other acute maladies of London over a series of years, their differences from year to year and from season to season, together with references to the prevailing weather, the whole body of observations being used to illustrate the doctrine of the epidemic constitution of the year or season, which he considered to depend often upon inscrutable telluric causes. The type of the acute disease varied, he found, according to the year and season, and the right treatment could not be adopted until the type was known.