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Before I conclude this effay, I would remark a peculiarity 

 jn the probability of teftimony which feems to add to it con- 

 iiderable force : This peculiarity is derived from the fucceflive 

 nature of the ads of the mind. In the material world caufes 

 .and efFedls co-exift ; and as we conjedure the connexion be- 

 tween them only from their correfpondence, it may frequently 

 be diiBcult to determine to which of two co-exifting objeds we 

 fhould afcribe an effed. The queftiori concerning phlogifton, 

 as ftated in Nicholfon's Firft Principles of Chemiftry, may afford 

 an example of fuch a difficulty. " The great queftion," accord- 

 ing to this writer," now is, whether inflammable air be con- 

 " tained in all combuftible bodies, (incc they do not all emit it 

 " by mere heat ; and it is evident, that if combuftion can be 

 " effeded without it in any one inftance, it cannot be the in- 

 "• difpenfable and univerfal principle of inflammability. Its 

 " exiftence is denied in fulphur, phofphorus, charcoal, metals, 

 " and fome other fubftances. It may however be obtained by 

 " heating thofe if water be prefent : whether it is afforded by the 

 '■'■ fub fiance under examination^ or by the water ^ is therefore thejuhje£i 

 " of controverfy." Book 2. fee. i. chap. 2. In the operations 

 of the mind this ambiguity cannot prevail in the fame degree. 

 We judge of the connedion of moral caufes and their effeds 

 by their order of fucceffion, and as the mind cannot at the fame 

 time give confiderable attention to more than one motive, we 

 are not liable to much ambiguity in our obfervation of the 

 tendency of that motive; befides, though in fome cafes feveral 

 motives may confpire to influence the mind to the fame deter- 

 mination, yet in others they operate Separately. In thofe other 

 cafes we may learn the natural tendency of thofe motives, and 

 we may apply the refult of thofe obfervations in cafes more 



complicated 



