 Botticelli produced "Fortitude" before loud protests from Pollaiolo and his brother Antonio that a second contract returned it to Piero and his studio produced the remaining six works in the series.In completing it, it is unclear how much of a contribution Antonio made to Piero's work and some art historians have attributed it instead entirely to Antonio.Billi, Albertini and Cruttwell argue from documents that the whole cycle was by Piero, whereas Ullman and others attribute all six works to Antonio based on stylistic comparisons with the few signed works by Antonio, including prints.