Contemporary physical cosmology in its inquiry into the structure of the universe has formulated hypotheses that posit unobservable empirically entities, which certainly have no possibility of being tested; such is the case of the multiverse cosmological hypothesis. This hypothesis is one of the most representative cases of speculative physical cosmology, and at the same time, is regarded by much of the scientific community as a scientific theory that could explain the origin of the universe and give an account of its laws and constants. Thus, it appears that science is an enterprise that can explain everything, while it has occupied the place of speculative philosophy in its attempt to elucidate reality to a general and ultimate level. From Nicholas Rescher’s philosophy such a condition for science is not possible, because science is not perfect and has limits. For Rescher, scientific rationality is determined by epistemological and ontological elements which demarcate its scientific character and scope.