 Finally, some (disputedly, see below) argue that explanations must be either natural (impersonal) or non-natural (personal).  8  &amp;  9  assert that if the contingent being identified in  1  is the universe, given that the universe encompasses all natural existents and the laws and principles governing them, the explanation must be in terms of a non-natural, eternal, necessary being that provides an intentional, personal, ultimate explanation. Since the argument proceeds independent of temporal considerations, the argument does not necessarily propose a first cause in time, but allows for a first or primary sustaining cause of the universe. As Aquinas noted, the philosophical arguments for God’s existence as first cause are compatible with the eternity of the universe (On the Eternity of the World), although he rejects the latter based on divine revelation.