IB Philosophy 10.8 the Republic, Books IV–IX – Plato Key Definitions

The Allegory of the Cave is Plato's famous image in Book VII of the Republic, depicting prisoners chained in a cave who mistake shadows for reality, illustrating the ascent of the soul from ignorance to knowledge of the Forms.

In Plato's hierarchy, the Form of the Good is the supreme Form described in Book VI of the Republic as what gives truth to objects of knowledge and the power of knowing to the knower, analogous to the sun that illuminates the visible world.

In Plato's republic, justice is not merely a legal or political concept but a condition of harmony in which each part of the soul or the state performs its proper function without interfering with the others, enabling the whole to flourish.

Legitimacy in political philosophy refers to the moral right of a government to exercise authority and the corresponding obligation of citizens to obey its laws. Theories of legitimacy range from consent-based accounts, which ground authority in the agreement of the governed, to procedural accounts that locate it in fair institutional processes, and substantive accounts that tie it to the government's delivery of justice and the common good.

The naturalistic fallacy is G.E. Moore's term for the error of defining the property of goodness in terms of any natural property such as pleasure, evolutionary fitness, or social approval. Moore argued that for any proposed natural equivalent of "good" one can always meaningfully ask whether that property really is good, suggesting that moral goodness is a simple, unanalysable quality irreducible to empirical description.

Periagoge is the Greek term for the turning or reorientation of the soul, which Plato uses to describe genuine education as a process of redirecting the soul's innate capacity for knowledge away from the sensible world toward the intelligible world of the Forms.

In Plato's Republic, the philosopher-king is a ruler who possesses genuine knowledge of the Forms and governs the ideal state on the basis of wisdom rather than popular opinion or personal ambition.

Political degeneration is Plato's account in Books VIII and IX of the Republic of how the ideal state (aristocracy ruled by philosopher-kings) progressively decays through four increasingly corrupt regimes: timocracy, oligarchy, democracy, and tyranny.

Soul-body dualism is Plato's view, developed across the Phaedo, Phaedrus, and Republic, that the soul (psyche) is a non-material entity distinct from the body (soma), capable of existing independently and oriented toward the intelligible world of the Forms.

Substance dualism, associated with Descartes, is the view that mind and body are two fundamentally different kinds of substance. The mind (res cogitans) is non-physical and indivisible, while the body (res extensa) is physical and divisible.

Plato's Theory of Forms holds that ultimate reality consists of eternal, unchanging, non-material entities called Forms, which serve as perfect paradigms of which sensible objects are imperfect copies.

The tripartite soul is Plato's theory that the human psyche consists of three distinct parts, reason, spirit, and appetite, each with its own desires, functions, and characteristic excellence.

The tyrannical soul is the most disordered psychological type in Plato's taxonomy, dominated by lawless eros (unbounded desire), in which the rational and spirited parts of the soul are entirely subjugated to the appetitive part's most extreme impulses.