Chance is often defined as the opposite of Necessity. The English word derives from the Latin cadere – to fall, especially cadens a fall, falling. Dictionary definitions refer to the fall of the dice, but the etymology suggests it is related to the grammatical idea of declension, which describes the falling or “leaning” away of the genitive, dative, and accusative cases from the “straight up” nominative case. The word connotes falling in the sense of decadence. Note the German for chance is Zufall.
Nothing occurs by chance (maton), but there is a reason (logos) and necessity (ananke) for everything.
The first thinker to suggest a physical explanation for chance in the universe was Epicurus. Epicurus was influenced strongly by Aristotle, who regarded chance as a fifth cause. He said there must be cases in which the normally straight paths of atoms in the universe occasionally bend a little and the atoms “swerve” to prevent the universe and ourselves from being completely determined by the mechanical laws of Democritus.
For Epicurus, the chance in his atomic swerve was simply a means to deny the fatalistic future implied by determinism (and necessity). As the Epicurean Roman Lucretius explained the idea,
…if all motion is always one long chain, and new motion arises out of the old in order invariable, and if the first-beginnings do not make by swerving a beginning of motion such as to break the decrees of fate, that cause may not follow cause from infinity, whence comes this freedom in living creatures all over the earth
(De Rerum Natura, Book 2, lines 251-256)
Epicurus did not say the swerve was directly involved in decisions so as to make them random. His critics, ancient and modern, have claimed mistakenly that Epicurus did assume “one swerve – one decision.” Some recent philosophers call this the “traditional interpretation” of Epicurean free will. On the contrary, following Aristotle, Epicurus thought human agents have an autonomous ability to transcend the necessity and chance of some events. This special ability makes us morally responsible for our actions.
…some things happen of necessity (ἀνάγκη), others by chance (τύχη), others through our own agency (παρ’ ἡμᾶς).
…necessity destroys responsibility and chance is uncertain; whereas our own actions are autonomous, and it is to them that praise and blame naturally attach.
λέγει ἐν ἄλλοις γίνεσθαι ἃ μὲν κατ’ ἀνάγκην, ἃ δὲ ἀπὸ τύχης, ἃ δὲ παρ’ ἡμᾶς, διὰ τὸ τὴν μὲν ἀνάγκην ἀνυπεύθυνον εἶναι, τὴν δὲ τύχην ἄστατον ὁρᾶν, τὸ δὲ παρ’ ἡμᾶς ἀδέσποτον, ᾧ καὶ τὸ μεμπτὸν καὶ τὸ ἐναντίον παρακολουθεῖν πέφυκεν
(Letter to Menoeceus, §133)
Despite abundant evidence, many philosophers deny that real chance exists. If a single event is determined by chance, then indeterminism would be true, they say, and undermine the very possibility of certain knowledge. Some go to the extreme of saying that chance makes the state of the world totally independent of any earlier states, which is nonsense, but it shows how anxious they are about chance.