The same must be true of every series of existences. Every series must have had a distinct self existent cause, or all existences must have had one and the same first cause. One first cause is sufficient, and it is unphilosophical to suppose more without evidence. The universe as a whole is a unit, and most philosophically attributed to the first cause. This cause is God.
All existences are necessary or contingent.
That existence or being is necessary whose non existence is naturally impossible. That existence is contingent whose non existence is naturally possible. Ideas of existences are necessary or contingent. That idea is necessary, the non existence of whose object, under the circumstances, cannot be conceived of as possible. That idea is contingent, the non existence of whose object may, under the circumstances, be conceived of as possible.