Al-Razi rejected the Aristotelian and Avicennian notions of a single universe revolving around a single world. He describes their main arguments against the existence of multiple worlds or universes, pointing out their weaknesses and refuting them. This rejection arose from his affirmation of atomism, as advocated by the Ash'ari school of Islamic theology, which entails the existence of vacant space in which the atoms move, combine and separate . He discussed more on the issue of the void – the empty spaces between stars and constellations in the universe, that contain few or no stars – in greater detail in volume 5 of the Matalib. He argued that there exists an infinite outer space beyond the known world, and that God has the power to fill the vacuum with an infinite number of universes.