The “Two Treatises of Government” (1690) offered political theories developed and refined by Locke during his years at Shaftesbury’s side. Rejecting the divine right of kings, Locke said that societies form governments by mutual (and, in later generations, tacit) agreement. Thus, when a king loses the consent of the governed, a society may remove him—an approach quoted almost verbatim in Thomas Jefferson’s 1776 Declaration of Independence . Locke also developed a definition of property as the product of a person’s labor that would be foundational for both Adam Smith’s capitalism and Karl Marx’s socialism.
Book II of the Essay sets out Locke's theory of ideas, including his distinction between passively acquired simple ideas, such as "red," "sweet," "round," etc., and actively built complex ideas , such as numbers, causes and effects, abstract ideas, ideas of substances, identity, and diversity. Locke also distinguishes between the truly existing primary qualities of bodies, like shape, motion and the arrangement of minute particles, and the “secondary qualities” that are "powers to produce various sensations in us" ( Essay, II. ) such as "red" and "sweet." These “secondary qualities,” Locke claims, are dependent on the “primary qualities.” This part of Locke’s thought would be sharply and famously criticized by Berkeley, who argued that there was no basis for a distinction between primary and secondary qualities and for asserting that primary qualities were any more “real” than the secondary ones. The weak point in Locke’s thought is that, in his own words, the substrate of those primary qualities, substance , is a “I know not what.” In other words, Locke is convinced that there must be something (substance) that is the foundation of objective existence and carries the primary qualities, but he is unable to further define it based on his empirical method.