8 Introduction . 
ABC will be equal to ALK - , that is , equal to two right Angles , which was to be demonftrated . 
And thus , by thefe three Inftances , may it eafily be perceived how differently the Mind is engaged in its Enquiries upon different Subjc & s - , for although all thefe admit of Certainty , yet it is come at by very different Means ; with ver ) ' good Rcafon fore they have by fomc been diitinguifhed into Hi - ftorical Certainty , Moral Certainty , and Demon - fixation , 'l'he firll depends upon the Credit and A - bilities of the Reporter ; the fécond , upon the ne - ceiTary Ideas of a good and perfect Being ; and the third upon the Teftimony of our Senfes , which only is our Guide in all Proportions relating to the tures and Operations of material Subitances , from the moil fimple and incompounded , to the moil tricate and abilrailed whatfoever . For whatfocver of this Nature is laid down for a Truth , let the thority of the Pcrfon be never fo great who nounces it , even fo far as to make me certain that he would not affirm any thing but what he knew to be true , and likewife would not by any Means lead me into an Error - , yet , until I go fomc fuch way to work as hath been taken in this lad Inilance , and come to fee the Rcafon and Neccflity of its being fo my felf from the fame Evidences , I cinnot be faid to know iï , or fliall I ever be able to make ufe of it to any Advantage , were it applicable to never fo many good Purpofes . 
This lafl Way of Inflruflion likewife , as it depends upon Dcmonilration , that is , upon fuch Evidences as cannot deceive , fo there are a great many Ways quently of proving the fame Thing , and all with equal Certainty , becaufc they all , in the fame Manner , keep clofe in View the fevcral Steps by which it is made appear •» as may be ihewn in the Propofition above - mentioned , which will admit of fevcral Ways

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