Agnosticism is the view that the truth values of certain claims—especially claims about the existence or non-existence of any deity, but also other religious and metaphysical claims—are unknown or unknowable. Agnosticism can be defined in various ways, and is sometimes used to indicate doubt or a skeptical approach to questions. In some senses, agnosticism is a stance about the difference between belief and knowledge, rather than about any specific claim or belief. In the popular sense, an agnostic is someone who is undecided about the existence of a deity or deities, whereas a theist and an atheist believe and disbelieve, respectively. In the strict sense, however, agnosticism is the view that human reason is incapable of providing sufficient rational grounds to justify the belief that deities either do or do not exist. Within agnosticism there are agnostic atheists (who do not believe any deity exists, but do not deny it as a possibility) and agnostic theists (who believe a deity exists but do not claim it as personal knowledge).