Full Idea
The Pythagoreans thought that the principles of mathematical entities were the principles of all entities.
Gist of Idea
Pythagoreans think mathematical principles are the principles of all of nature
Source
report of Pythagoras (reports [c.530 BCE]) by Aristotle - Metaphysics 985b
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.19