The Scottish philosopher Thomas Reid (1710-1796) first published Essays on Active Powers of Man in 1788 while he was Professor of Philosophy at King's College, Aberdeen. The work contains a set of essays on active power, the will, principles of action, the liberty of moral agents, and morals. Reid was a key figure in the Scottish Enlightenment and one of the founders of the 'common sense' school of philosophy. In Active Powers Reid gives his fullest exploration of sensus communis as the basis of all philosophical inquiry. He uses common sense realism to argue for the existence of a stable external world, the existence of other minds, and to offer a powerful challenge to versions of the Theory of Ideas advocated by Hume (1711-1776) and Locke (1632-1704). This is a key work of the Scottish Enlightenment that made important contributions to fundamental debates about the basis of philosophical inquiry.