THE CAUSATION OF DISEASE. 339 



differs considerably, how much greater reason is there that 

 diseases should be variable ? 



2. Where each specific form of mal-E is constant, the S varying. 

 — As an example of a theoretically fixed mal-E, let us take a 

 chemical poison — say, prussic acid. Now, since the S is in no two 

 cases the same, it follows that the resulting disorder — prussic 

 acid poisoning — can never be exactly the same either. It may be 

 thought that this is mere hair-splitting ; but I maintain that 

 it is simply logical, and consistent with the admissions we have 

 been compelled to make. Suppose that the resulting disturb- 

 ance were entirely different in the two cases, should we still be 

 justified in regarding the disease as at all the same, even though 

 due to the same amount of the same poison ? In prussic acid 

 poisoning the effects would, I admit, probably be very much the 

 same in all cases ; but we know that different individuals differ 

 very considerably in their mode of response to different kinds 

 of poisons — and this shows us that the nature of the E is not an 

 absolute criterion of the nature of the disorder, and hence, as 

 we have seen, one reason why we cannot classify diseases 

 according to their external causes. In the case of chemical 

 poisons, the specific mal-E no doubt forms a valuable means of 

 classification — for practical purposes at all events — and when 

 called to a patient suffering from prussic acid poisoning we 

 should not enter into a disquisition on the nature of the morbid 

 changes, bat, knowing prussic acid to be the disturbing agent, 

 should proceed with all promptitude to take such steps as are 

 known to be useful in counteracting its effects. 



Seeing, then, that even if, on the one hand, the S were 

 .always constant, or if, on the other, E were always constant, 

 diseases would still vary very considerably in type, and seeing, 

 moreover, that, as a matter of fact, both S and mal-E are 

 variable, the truth that disease types are variable becomes at 

 once obvious. 



Variability in S, perhaps even more than variability in E, is 

 a cause of variability in disease. No doubt there is a certain 

 likeness in all individuals of the same species in respect of 

 their response to specific mal-E's, and since these latter also 



