Petitio principii

Petition principii, or begging the question, is a logical fallacy where a claim is supported by a premise that either restates the claim or assumes it to be true. Essentially, it tries to prove a proposition by taking the proposition for granted.

When this fallacy involves only one variable, it is sometimes called hysteron proteron (Greek for 'later earlier'), a rhetorical device. An example is the statement:

'Opium induces sleep because it has a soporific quality.'

The word 'soporific' means 'causing sleep,' so the phrase doesn't actually explain why opium causes sleep. A sentence that properly explains why opium induces sleep might be:

'Opium induces sleep because it contains Morphine-6-glucuronide, which inhibits the brain's pain receptors, causing a pleasurable sensation that eventually leads to sleep.'

This fallacy can occur in a simple statement or through a more complex series of statements that circle back to the original claim, thus 'proving' it without providing any actual evidence.