I. All Propositions that involve any thing of Obscurity, are to be proved without alledging any thing in proof of them, besides Definitions which have gone before, or Axioms that have already been granted, or Propositions before demonstrated, or the Construction of the thing in discourse, when any Operation is to be done.
II. VVe are never to make an ill use of the Equivocation of Terms. This may be occasioned by not adding their Definitions, to restrain or ex∣plain them. For to every true Demonstration two things especially are required. First, That in the Matter it self nothing be contained, but what is certain and indubitable: The Other, That there be no fault in the Form of Argumentation: Which Faults we shall avoid by the Rules now given, that is, by not making an ill use of Equivocal Terms, and by using no other Propositions in our Demon∣strations, but what are either Definitions of Terms before explained, or Axioms that have been before granted, and which ought not to have been sup∣posed, without being sure of their Evidence; or Propositions, that have been before demonstrated, or the Construction of the Thing of which we treat, when any operation is to be performed. For by observing of these things all Defects or Mistakes will be easily avoided both in Matter and Form.
As to the Constitution of Demonstrations, we have handled it in the Nineteenth Chapter of the Third Part.