292 THE CAUSATION OF DISEASE. 



If the S is so constituted that healthy vital inter-action is im- 

 possible under any conditions of E, that S is absolutely and 

 inevitably abnormal ; and conversely ', if an E is such that no S 

 can respond healthily to it, that E is absolutely and inevitably 

 abnormal. But if a set of conditions exists in which the S can 

 act healthily, that S must be regarded, as possessing, at all events, 

 a potential normality ; and conversely, if there is a set of condi- 

 tions to which some individuals are capable of responding healthily, 

 that E is normal to them. 



In order to show how normality of S and E can only be 

 considered in relation to one another, let us take, again, the 

 instance of a coloured tribe living healthily in an ague- 

 stricken district. Here S + E results in perfect health, and, 

 therefore, both are normal — the S on the one hand, and the E 

 (including the ague poison) on the other ; for this being in- 

 capable of striking disease into these black people, it is not 

 abnormal to them, and therefore must be normal. Let us 

 further take the case of a .number of white men living healthily 

 amid an ordinary civilized E — say, in England. Here again 

 both the S and the E are normal — health being the criterion 

 of normality for each. But now let the white and the black 

 change places : the former would quickly sicken in the ague- 

 stricken district, and the like would in a large degree be the 

 fate of the black suddenly transplanted to a civilized E. The 

 black might, with some show of justice, exclaim : " These 

 white men are unhealthy creatures, for they sicken and die in 

 this district where we thrive so well ; " while the white, with 

 equal justice, might retort : "You blacks are poor, sickly, 

 unhealthy creatures, for you rapidly sicken under this healthy 

 E of ours." Hence this paradox — the structure of each is 

 both normal and abnormal ; each is normal to its own par- 

 ticular E, but abnormal to the other. The clue, as I say, to 

 the situation is this : normality of S has no meaning by itself, 

 and can only be considered in relation to E. Every individual 

 who can live healthily in a particular E is structurally normal 

 as regards that E, and there is no fixed normality for all. The 

 same is true of the several forms of E. E must always be 

 considered in relation to S, and any E is normal to a par- 

 ticular S if it permits that S to live healthily amid it. Thus 

 an ague-engendering E may be normal to some, but virulently 



