446 INTRODUCTORY LECTURES. 



of nature, or to the acquiring a knowledge of any enumeration 

 of things that are to be distinguished from one another. 



There have been objections to the application of method to 

 some parts of science, but even those who make such objections, 

 agree that, wherever the species of any kind are very numerous, 

 it is impossible to prosecute the investigation with clearness and 

 advantage without method. If there are ten thousand different 

 species of plants, or five thousand different species of insects to 

 learn, to acquire the knowledge of all these separately, as was 

 the state of things among the ancients, is next to impossible ; 

 and we know that we can now acquire the knowledge of ten 

 thousand more easily than we could formerly the knowledge 

 of five hundred. 



But, from the same means by which we have acquired the 

 knowledge of method in general, I must suppose you to have 

 acquired it for the purpose I speak of; and I go on to say, 

 that the distinguishing of things by genus and species is uni- 

 versally applicable to every two things that are to be distin- 

 guished from one another, and, therefore, among others, to the 

 distinguishing of diseases ; and this distinction only requires 

 that we should know what is peculiar to one thing, and at the 

 same time common to no other ; it is, to know every thing by 

 something peculiar to it and in common to no other. These se- 

 veral circumstances or particulars, and the combination of them 

 that serve this purpose, is what in natural history we call a 

 character ; in physic we call it a Pathognomonic. These were 

 long ago marked, indeed, as the great desiderata in the history 

 of diseases : they long talked of pathognomonics in the history 

 of diseases, in order to their accurate distinction, but it was, 

 however, in very few diseases that such pathognomonics were 

 found out ; where a single symptom proved pathognomonic, it 

 was perhaps observed, but this does not occur once in a hundred 

 cases : it is a combination of several symptoms that is necessary 

 to constitute such a character or pathognomonic, and this was 

 not attempted. 



In all the ancient systems, and I may say till very lately, they 

 had a title, the Diagnostic, which should have given the pathog- 

 nomonics the strict and proper characters ; but if you consult 

 these you will find, or a little acquaintance with physic will 



