Form of the Good in the context of Theory of Forms

The Megarian school of philosophy, which flourished in the 4th century BC, was founded by Euclides of Megara, one of the pupils of Socrates. Its ethical teachings were derived from Socrates, recognizing a single good, which was apparently combined with the Eleatic doctrine of Unity. Some of Euclides' successors developed logic to such an extent that they became a separate school, known as the Dialectical school. Their work on modal logic, logical conditionals, and propositional logic played an important role in the development of logic in antiquity.

Summum bonum is a Latin expression meaning the highest or ultimate good, which was introduced by the Roman philosopher Cicero to denote the fundamental principle on which some system of ethics is based—that is, the aim of actions, which, if consistently pursued, will lead to the best possible life. Since Cicero, the expression has acquired a secondary meaning as the essence or ultimate metaphysical principle of Goodness itself, or what Plato called the Form of the Good. These two meanings do not necessarily coincide. For example, Epicurean and Cyrenaic philosophers claimed that the 'good life' consistently aimed for pleasure, without suggesting that pleasure constituted the meaning or essence of Goodness outside the ethical sphere. In De finibus, Cicero explains and compares the ethical systems of several schools of Greek philosophy, including Stoicism, Epicureanism, Aristotelianism and Platonism, based on how each defines the ethical summum bonum differently.

The term was used in medieval philosophy. In the Thomist synthesis of Aristotelianism and Christianity, the highest good is usually defined as the life of the righteous and/or the life led in communion with God and according to God's precepts. In Kantianism, it was used to describe the ultimate importance, the singular and overriding end which human beings ought to pursue.

Speusippus (/spjuːˈsɪpəs/; Ancient Greek: Σπεύσιππος; c. 408 – 339/8 BC) was an ancient Greek philosopher. Speusippus was Plato's nephew by his sister Potone. After Plato's death, c. 348 BC, Speusippus inherited the Academy, near age 60, and remained its head for the next eight years. However, following a stroke, he passed the chair to Xenocrates. Although the successor to Plato in the Academy, Speusippus frequently diverged from Plato's teachings. He rejected Plato's Theory of Forms, and whereas Plato had identified the Good with the ultimate principle, Speusippus maintained that the Good was merely secondary. He also argued that it is impossible to have satisfactory knowledge of any thing without knowing all the differences by which it is separated from everything else.

The standard edition of the surviving fragments and testimonies is Leonardo Tarán's Speusippus of Athens: A Critical Study with a Collection of the Related Texts and Commentary (1982).