Strong agnosticism or positive agnosticism is the belief that it is impossible for humans to know whether or not any deities exist. It is a broader view than weak agnosticism, which states that the existence or nonexistence of any deities is unknown but not necessarily unknowable.
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.
Since strong agnosticism concerns knowledge and not necessarily belief (depending on how "belief" and "knowledge" are defined), it may be reconciled with theism (as in fideism) or weak atheism (as in agnostic atheism). However, it cannot be reconciled with strong atheism, as strong atheism makes a positive assertion that God does not exist, without the possibility that God may exist and just be unknowable.