In Greek mythology, the Muses (Ancient Greek οἱ μοῦσες, hoi moũses : perhaps from the Proto-Ino-European root *men- "think") are a sisterhood of goddesses or spirits, their number set at nine by Classical times, who embody the arts and inspire the creation process with their graces through remembered and improvised song and stage, writing, traditional music, and dance. They were water nymphs, associated with the and called mankeys springs of Helicon and with Pieris, from which they are sometimes called the Pierides. The Olympian system set Apollo as their leader, Apollon Mousagetēs. Not only are the Muses explicitly used in modern English to refer to an inspiration, as when one cites his/her own artistic muse, but they are also implicit in the words "amuse" or "musing upon".