I consider time as flowing or increasing by continual flux & other quantities as increasing continually in time & from ye fluxion of time I give the name of fluxions to the velocitys wth wch all other quantities increase. ...I expose time by any quantity flowing uniformly and represent its fluxion by an unit, & the fluxions of other quantities I represent by any other fit symbols... This Method is derived immediately from Nature her self.
Isaac Newton, draft review of Commercium epistolicum (1712) The Mathematical Papers of Isaac Newton (1969) Vol. 3, ed. D. T. Whiteside, p. 17.