PREFACE. ix 



There are fome modern praftitioners, who declaim a- 

 gainft medical theory in general, not confidering that to 

 think is to theorize ; and that no one can direft a me- 

 thod of cure to a perfon labouring under difeafe without 

 thinking, that is, without theorizing ; and happy there- 

 fore is the patient, whofe phyfician poffefies the beft 

 theory. 



The words idea, perception, fenfation, recolle&ion, 

 fuggeftion, and aflbciation, are each of them ufed in this 

 treatife in a more limited fenfe than in the writers of 

 metaphyfic. The author was in doubt, whether he 

 ihould rather have fubftituted new words inftead of 

 them ; but was at length of opinion, that new defini- 

 tions of words already in ufe would be lefs burthenfome 

 to the memory of the reader. 



A great part of this work has lain by the writer above 

 twenty years, as fome of his friends can teftify : he had 

 hoped by frequent revifion to have made it more worthy 

 the acceptance of the public ; this however his other 

 perpetual occupations have in part prevented, and may 

 continue to prevent, as long as he may be capable of re- 

 vifing it ; he therefore begs of the candid reader to ac- 

 cept of it in its prefent ftate, and to excufe any inaccu- 

 racies of expreffion, or of conclufion, into which the intri- 

 cacy of his fubjeft, the general imperfeUon of lan- 

 guage, or the frailty he has in common with other men, 

 may have betrayed him ; and from which he has not the 

 vanity to believe this treatife to be exempt. 



VOL, I. 



