Rambam says explicitly in his introduction to Shemoneh Perakim that he learned from non-Jewish writings. There is wisdom among the other nations. Sometimes they even say more or lessthe same as rabbinic sources. For example, John Adams said that the US Constitution is only appropriate for a religious and moral people, The Netziv says in He’emek Devar that the mitzva to appoint a king was conditioned on the people asking for one because some generations need a king and some need a republican form of government. Great minds really do think alike.
L’havdil