The inquiry into the ideas that dominate educational theory and policy. The central questions are what it is worthwhile or necessary to teach, and what are the best ways of doing it. Different views about human nature will influence answers to both questions. Rival metaphors include that of the young mind as an empty pot that needs filling up, versus the mind as an unexercised athlete that needs practice. The focus has often been Plato's question whether virtue can be taught, and this quickly spreads to include intellectual virtue. Plato's paradox from the Meno that you cannot understand what you do not know and therefore cannot set about discovering it is usually felt to be soluble (see learning paradox), but in practice education remains bedevilled by the fact that people missing it do not know what they are missing. The solution from the right is coercion and discipline; that from the left is to worry whether education involves discrimination, or indeed trespasses against the equal rights and dignity of the ignorant and stupid. Eminent philosophers who have written at length about education include Plato and Aristotle, Locke, Rousseau and Mill, and in the twentieth century, Dewey.