Sentence:  ... Geometry, "an excellent Logic," as you obſerve, "where the definitions are clear, where the Poſtulata cannot be refuſed, nor the <WSD>Axioms</WSD> denied"; ... 
Meaning of the word marked with <WSD> tags:  A fundamental assumption that serves as a basis for deduction of theorems; a postulate (sometimes distinguished from postulates as being universally applicable, whereas postulates are particular to a certain science or context).