Strong agnosticism is usually justified on the epistemological grounds that humans can only experience the natural world and thus cannot know about anything which may exist outside it, including deities. One criticism is that this justification is only valid if deities are viewed as exclusively supernatural beings, but to support such a view one must have at least some knowledge of the nature of deities. The agnostic reply is, as the natural world can be explained by science, the defining feature of any deity must be supernatural.