 Arguments in this vein are more firmly planted in empirical, a posteriori reflection than the ontological argument, but some versions employ a priori reasons as well. There are various versions. Some argue that the cosmos had an initial cause outside it, a First Cause in time. Others argue that the cosmos has a necessary, sustaining cause from instant to instant, whether or not the cosmos had a temporal origin. The two versions are not mutually exclusive, for it is possible both that the cosmos had a First Cause and that it has a continuous, sustaining cause.