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impaired by the former, because the former is related with the 

 properties engaged in the dependence; not related with the latter, 

 because although they produce a certain affection of the seat of 

 the properties which animate the respiratory organs, they do not 

 produce a change in the nature and relations of these properties, 

 to expect which would be like expecting that paralysis of nerves 

 should not occur from pressure upon a part of the brain, because it 

 still retains some properties of life. 



8. It has been stated that related disease happens in two 

 ways, which may here be repeated: 1st, by disturbing an habitual 

 relation of the regular dependent kind; 2nd, by a new relation 

 which is opened between parts not before connected by intercourse 

 of function, in consequence of a new condition which one of them 

 has assumed; it has been stated (and examples given) that disease 

 might occupy a succession of seats without the existence of any 

 causative relation between them. It is necessary before we pro- 

 ceed any further to inquire after the method of distinguishing be- 

 tween diseases which, though occurring in a series, are independent 

 of each other, and those in which the subsequent, is produced by 

 the preceding disease. 



9. In making this distinction we are liable to frequent error: 

 the only grounds of the distinction, however, are as follow. Mere 

 succession, as has before been insisted, can never prove causation : 

 but it indicates causation, from the analogy of succession to those 

 palpable instances of causation in which the dependence of the effect 

 upon the assigned cause may be proved by the result of analysis and 

 of synthesis: of taking a way (or witholding), and of combining the 

 causes. Succession then, upon this ground of analogy, which has 

 been more fully explained, may indicate causation; and yet we do 

 not suffer every instance of succession to suggest ever so faintly 

 an inference of causation. 



10. The succession of an effect to its true cause is invaria- 

 ble! from analogy in this respect, we infer positively the operation 

 of a cause in all instances of invariable succession; thus day and 

 night invariably succeed the presence or absence of the sun. But 

 we presume still further upon this analogy, we infer the operation 

 of a cause, when the succession of the same consequence to the 

 same antecedent is frequent, but not invariable; thus, an ounce 

 and a half of laudanum taken into the stomach will commonly, but 

 not always, produce death; we have no hesitation in assigning the 

 laudanum as the cause of death, in those instances in which death 

 takes place, notwithstanding there are other instances where the 

 obvious circumstances are alike, in which it is not followed by 

 death. "Invariable succession bears so strong an analogy to causa- 

 tion, that we scarcely suspect the possibility of oilr being deceived 

 in an inference grounded upon it; and yet we do sometimes make 

 a false inference founded upon past invariable succession, as is 

 proved by additional, or subsequent experience. Frequent suc- 

 cession of like to like, bears an analogy to the- invariable, and 



