Science is simply common sense at its best, that is, rigidly accurate in observation, and merciless to fallacy in logic.

—Thomas Huxley

To be a critical thinker, any belief you hold should be justified by facts or sound argument. Any argument that is supported by unsound reason is referred to as a logical fallacy.

Fallacies are often used unintentionally due to ignorance but fallacies can also be used deliberately in order to manipulate others into accepting a certain position.

Bare Assertion Fallacy

Claiming something is true without justifying that claim is committing the most basic and prolific logical fallacy: the Bare Assertion.

Note that simply stating a known fact without justification is not committing this fallacy. Commonly known facts do not carry a burden of proof.