The German mathematician Martin Eichler once stated that there were five fundamental operations of mathematics: addition, subtraction, multiplication, division, and modular forms. This was also mentioned in Simon Singh's book on Fermat's last theorem.
I don't know much about modular forms beyond the Wikipedia entry, and am quite confused by this remark. The usual operations seem to be different entities (two numbers input, one number output) than a modular form (one number input, one number output).
Does anyone have some deeper insight into Martin Eichler's assertion?