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the absence of calomel, because this also might cure fever; or that 

 pleurisy consists in a deficiency of nitre, or in the want of a blister 

 on the skin; or, pursuing the direct principle of causation, that 

 pneumonia happens from excess of blood, because it may be cured 

 by repeated bleedings: we cannot attribute the disease to such a 

 cause, seeing that the pneumonia might happen the day after a 

 pint of blood has been taken from the arm, at which time there is 

 a pint of blood less in the body than existed in it a week before, 

 when pneumonia was not present; or that apoplexy is produced 

 by excess of blood, seeing that fatal apoplexy may take place in a 

 subject who has been bled twice a week for a month previously, 

 and lived upon gruel; there must certainly be less blood in the 

 body at the end of such a month than there was two mouths before, 

 when the subject was in good health and in habits of full repletion. 

 If it be said these diseases are produced by a local excess of blood, 

 I ask what occasions such local excess? it must be answered, pre- 

 vious disease: such illustration may be greatly extended r To 

 this may be added examples of the following kind: a person having 

 a pain in the head with a furred tongue, may become free from 

 the pain in the head and the tongue might become clean, after 

 losing twelve ounces of blood from the back of the neck by 

 cupping, without the use of any other efficient remedy: the same 

 result, in a case of a similar character, may be produced by 

 emetics; the same by bleeding from the arm, or from the temples 

 by leeches, or bydigitilis; the same from aloes, antimony, blue 

 pill; by an issue; or a blister on the scalp; by change of air; 

 perhaps by laudanum, &c. The inference from these facts (and 

 there are hundreds of such) is, that the same results might be ac- 

 complished by different means. If the state of disease is changed 

 and the condition of health restored by agents which supply pre- 

 risely those properties which are deficient, or remove those precise 

 ones which are in excess, how happens it that the agents producing 

 this similitude of effects are so greatly diversified? 



4. The question, how many different agents produce the same 

 effect? may be answered thus, by one determined to support the 

 doctrine of the direct causation : " All these various remedies may 

 have some precise properties in common, with which alone the 

 state of disease is related; these properties being allied with diffe- 

 rent substances, may appear to form a multiplicity of means of ac- 

 complishing one purpose, while in reality the efficient properties 

 are alike and common to them all." This observation is perfectly 

 just, and unanswerable in some other cases of causation: thus, a 

 bullet may be propelled from a tube either by the force of air or of 

 gunpowder, or perhaps steam; these three are different, but act 

 by a common property, viz. a force of expansion with which the 

 effect is related. Thus, also, ipecacuanha, antimony, zinc, &c. 

 are capable of exciting the action of vomiting, which would not 

 take place unless these substances contained a common property 

 which identified the state necessary to vomiting; thus, alio, toe 



