Dikē is the old Greek word for "law, justice." By the fourth century BCE it was largely replaced by its cognate dikaiosynē, Plato's cardinal virtue, justice.
In early Greece (Homer, Hesiod), dikē ranges in meaning from a specific claim by one party to a dispute, to a judgment or settlement, or to the personified force or goddess Justice/Law. In Homer's Iliad, the trial scene on Achilles's shield (18.497–508) depicts the elders (as judges) in a competition to see who can propose the straightest dikē (the best judgment/settlement). In Hesiod's Works and Days animals eat one another, but Zeus gave humans dikē —law, judicial process—which is far better (276–280), and Dikē sits beside her father Zeus and punishes those who corrupt the judicial process with crooked dikē (256–262).
The sixth-century lawgiver Solon promotes dikē —law-abiding conduct—as part of a general program of eunomia (good order, law and order). He also speaks of his legislation as providing a straight dikē (judicial process) for every Athenian. For the fifth-century thinker Heraclitus, dikē becomes a cosmic force of order and balance. Heraclitus's dikē is not static, however, but—as in a lawsuit—a balance of opposing forces, so that, as he says paradoxically, dikē is eris (strife).
Fifth-century tragedians regularly see dikē as a cosmic force, justice, largely in the sense of punishment or retribution. All the characters in Aeschylus's Oresteia claim to seek dikē —justice—primarily in the sense of punishment or revenge for previous wrongs, though in some passages the chorus suggest a larger sense of justice as cosmic and social order. Plato's Protagoras pictures the sophist Protagoras telling a story in which the gods give dikē, law or justice, together with aidōs (respect) to all humans; he concludes from this that dikē is necessary for the survival of human society.
Through the fifth century, dikē in all its meanings—from judicial process to cosmic force—remains something external to human beings. Not until the fourth century does Plato make justice a personal virtue of individuals, and then it is no longer dikē but dikaiosynē (see especially Republic, Book IV).
Gagarin, Michael. "Dikē in the Works and Days." Classical Philology 68 (1973): 81–94. Argues that in the beginning meanings of dikē are restricted to the realm of law and the legal process.
Havelock, Eric A. The Greek Concept of Justice. Cambridge, MA: Harvard University Press, 1978. Argues that in a preliterate, oral culture, justice is primarily procedure.
Lloyd-Jones, Hugh. The Justice of Zeus. 2nd ed. Berkeley: University of California Press, 1983. Broad discussion of divine justice—often seen as equivalent to dikē —in Greek literature.
The works of Hesiod, including his poem Works and Days, are available in the Penguin series and several other translations.
Michael Gagarin (2005)