Knowledge is an awareness of facts, a familiarity with individuals and
situations, or a practical skill. Knowledge of facts, also called
propositional knowledge, is often characterized as true belief that is
distinct from opinion or guesswork by virtue of justification. While there is
wide agreement among philosophers that propositional knowledge is a form of
true belief, many controversies focus on justification. This includes
questions like how to understand justification, whether it is needed at all,
and whether something else besides it is needed. These controversies
intensified in the latter half of the 20th century due to a series of thought
experiments called Gettier cases that provoked alternative definitions.