Symbolical algebra is … the science which treats of the combination of operations defined not by their nature, … but by the laws of combination to which they are subject....[W]e suppose the existence of classes of unknown operations subject to the same laws.

Gregory (1865) The Mathematical Writings of Duncan Farquharson Gregory, M.A., Late Fellow of Trinity College, Cambridge, Robert Leslie Ellis ed. p. 2, as cited in: Patricia R. Allaire and Robert E. Bradley. "Symbolical algebra as a foundation for calculus: DF Gregory's contribution." Historia Mathematica 29.4 (2002): p. 404