Xenophanes, the founder of the Eleatic school of philosophy, born in Asia Minor; was the first to enunciate the doctrine “all is one,” but “without specifying,” says Schwegler, “whether this unity was intellectual or moral.... Aristotle says he called God the one.” See Eleatics.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Xenocrates * Xenophon