The purely formal sciences, logic and mathematics, deal with those relations which are, or can be, independent of the particular content or the substance of objects. To mathematics in particular fall those relations between objects which involve the concepts of magnitude, of measure and of number.

Theorie der Cornplexen Zahlensysteme insbesondere der gemeinen, imaginären Zahlen und der Hamilton'schen Quaternionen nebst ihrer, geometrischen Darstellung (p. 1), Leopold Voss, Leipzig, Germany. 1867