Monism

Monism is a philosophy and a metaphysical stance that everything in the universe is originally comprised of one thing or is controlled by one principle.

Variantions
Monism had many variations during Ancient Greece:
 * Thales believed that everything had originated from water.
 * Anaximander believed that everything had originated from Apeiron, which is existent, indefinent and eternal.
 * Anaximenes believed that everything had originated from air.
 * Heraclitus believed that everything had originated from fire, which also means the 'change'.

Monism had later influenced Pythagoreanism, which believed that everything is a manifestation of mathematics. Monism had also later expanded to theory of classical elements, which refers to fire, water, soil, air.

In short, Monism largely influenced not only philosophy but also mathematics, physics, chemistry and science in general. The question on the source of everything in universe is still unanswered as wether what is comprising elementry particles is unknown.