6 Introduction . 
that in the whole Affair he mull proceed by their E - videncesonly : To thisPurpofe therefore , he defcribes Angles to be view'd by the Eye , and learns how to meafurc and compare them with one another , until at lall , by the Evidences of his Senfes only , he is be» yond all Doubt convinced , that the three Angles of any Triangle whatfoever , are equal to two right gles . And in any Propofition of this Nature , though after fome Time a Perlon comes to be lo venant with the Terms ufed therein , that he may frequently be able to apprehend theRcafon and Truth of it , without piéluring of it to his Sight by a gram i yet the Mind all the Way imagines it , and keeps the Picture of the Thing fpokc of , clofe in View - , for no longer than he does io , can he difcern his Evidences ; and confequently without it can never make any certain Conclufion . 
But bccaufe by this lall Inftance , I lhall endeavour chiefly to illultratc what is meant by mechanical Knowledge , or phyfical Certainty , I would herein be as particular and plain as pollible , and therefore lhall go through the leverai Steps whereby the Mind comes at the Truth of this Propofition i which al - , though it may be done various Ways , yet they will agree in this , that they keep always clofe to the fame Évidences . 
i . To know clearly what is meant by a right gle , let the Perpendicular FG , Fig . i . be let fall upon the right Line XZ , and it makes two right Angles M N . 
• . If then the Perpendicular FG be drawn through XZ , as in Fig . 2 . there will be made four right Angles , MNOP . 
3 . Farther , If the Perpendicular FG be changed into a Line which cuts the other XZ , ly , as . in Fig . 3 , there will be four Angles made 
thereby i

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