Ancient Greek philosophers distinguished between demonstrated knowledge and opinion. Plato distinguished between two levels of awareness: opinion and knowledge. According to him, opinions are based on sensations and are impermanent; knowledge on the other hand is timeless and based on essences. In The Republic, these concepts were illustrated using the metaphor of the sun, the divided line, and the allegory of the cave.
Today, Plato's analogy of the divided line is a well-known illustration of the distinction between knowledge and opinion, or knowledge and belief. Opinions can be persuasive, but have to be defended by facts, and assertions, whether true of false.
Read a paper on Plato's Analogy of the Divided Line.
Another interesting reading is available as the Platonic Epistemology and the Nature of Philosophical Activity: A Comparison with Indian Philosophy.