It is only the inner genetic co-ordination of the two, viz. the internal and the external history (for Disease has also an external history) that can conduct to the _actual History_ of the Disease. This may be defined as _a genetic co-ordination and statement of the symptoms of a Disease under different conditions and in different individuals, from the first moment at which they arose and came under observation down to the time when the report is made_; or, expressed more briefly, the History of a Disease is _a genetic co-ordination and account of its development and progress in time_ (as conditioned by time). Supposing Time, Relations, and Number of individuals definitely limited, a Special History is the result; while the General History of a Disease properly speaking can _never_ be viewed as isolated from its surroundings. In that case the conditions on which the generation and origin of the particular Disease depend would necessarily cease entirely and for ever to exist. Now if we analyse the conception of the History of a Disease into its component parts, we shall get to know its special _contents_, the efficient factors of which it is compounded, and which the Historian has to comprehend and express. The function of History is to exhibit something that has happened; naturally therefore the first thing the Historian must do is to look out for the point of time at which the process of change began. But certain generating factors and influences are indispensable to every process of change, and their activity again is dependent on certain favourable external conditions; and so it becomes the next duty of the Historian to authenticate the existence of the said favourable influences as well as of the generating factors, and concurrently to determine in what manner they came into active operation. Inasmuch as it happens however sometimes that the interposing or favouring as well as the generative factors are known to be present, and yet no outbreak of disease occurs, so far as we see, or only an incompletely developed one, those influences also will require authentication which hindered or modified the potential activity of the factors.