But if the subject of discussion or _suppositio_ be Mythology or Heraldry, such propositions as the above are to the purpose, and form legitimate pairs of Contradictories.
In Formal Logic, in short, we may make at discretion any assumption whatever as to the existence, or as to any condition of the existence of any particular term or terms; and then certain implications and conclusions follow in consistency with that hypothesis or datum.

Still, our conclusions will themselves be only hypothetical, depending on the truth of the datum; and, of course, until this is empirically ascertained, we are as far as ever from empirical reality.

(Venn: _Symbolic Logic_, c.

6; Keynes: _Formal Logic_, Part II.c.

7: _cf._ Wolf: _Studies in Logic_.).