The argument from necessary being starts with the observation that things in the world have contingent existence, meaning that they exist but do not have to exist. However, Aquinas argued that there must be at least one being that has necessary existence, meaning that it exists necessarily and cannot not exist. This necessary being, according to Aquinas, is God. The argument from first efficient cause starts with the observation that there are things in the world that are caused or brought into existence by other things. However, Aquinas argued that this chain of causation cannot go on infinitely. There must be a first efficient cause, a cause that itself is not caused by anything else.