How do you know they exist?
According to Avicenna, the universe consists of a chain of actual
beings, each giving existence to the one below it and responsible for
the existence of the rest of the chain below. Because he deems an
actual infinite impossible, the chain as a whole must terminate in a
being that is wholly simple and one, whose essence is its very
existence and therefore is self-sufficient and not in need of
something else to give it existence. Because its existence is not
contingent on or necessitated by something else, but necessary and
eternal in itself, it satisfies the condition of being the
necessitating cause of the entire chain that constitutes the eternal
world of contingent existing things. Thus, Avicenna's ontological
system rests on the conception of God, that is the First Cause, as the
Wajib al-Wujud (necessary existent). There is a gradual multiplication
of beings through a timeless emanation from God as a result of his
self-knowledge. This was the first attempt at using the method of a
priori proof, which utilizes intuition and reason alone.
http://en.wikipedia.org/wiki/Ontological_argument
