Those influences which have the power of acting upon the organism in such a way as to excite disease, especially in the system which is already predisposed, were formerly divided into cognizable and non-cognizable: the first section including all agencies the existence of which could be recognized independently of their action in producing disease, such as heat and cold; and the second referring to influences which were not recognizable, but the existence of which was inferred from the effects produced. In this second class all contagious and infectious influences, endemic or epidemic, were included. The system of classification of half a century ago has now become obsolete, but it is worth while to refer to it, because although comparatively recent investigations have proved beyond doubt that the cause of some of the most virulent infectious disorders are quite cognizable, there are other maladies which are endemic, epidemic, and in some cases contagious, which arise from causes the existence of which can only be inferred from the effects produced. The causes which are cognizable or apparent to the senses are mechanical and chemical agencies, food, exertion, excitement, excessive and defective secretion, defective ventilation and drainage, and climatic changes; and it is evident that several of these agencies, which have already been considered, are capable of acting both as predisposing and exciting causes - exciting when they become sufficiently intense to produce the disease to which their primary action only rendered the organism susceptible. Thus, errors in diet, excitement without exertion, excessive secretion, defective secretion, impure atmosphere, and changes of temperature may all act as exciting causes of disease. While they are placed among the predisposing causes - in fact whether the causes are mechanical or chemical in their character - they may be alternately predisposing and exciting influences.