An Essay Concerning Human Understanding is a work by John Locke concerning the foundation of human knowledge and understanding. It first appeared in 1689 (although dated 1690) with the printed title An Essay Concerning Humane Understanding. He describes the mind at birth as a blank slate (tabula rasa, although he did not use those actual words) filled later through experience. The essay was one of the principal sources of empiricism in modern philosophy, and influenced many enlightenment philosophers, such as David Hume and George Berkeley. Book I of the Essay is Locke's attempt to refute the rationalist notion of innate ideas. Book II sets out Locke's theory of ideas, including his distinction between passively acquired simple ideas—such as "red," "sweet," "round"—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" such as "red" and "sweet." These secondary qualities, Locke claims, are dependent on the primary qualities. He also offers a theory of personal identity, offering a largely psychological criterion. Book III is concerned with language, and Book IV with knowledge, including intuition, mathematics, moral philosophy, natural philosophy ("science"), faith, and opinion. Content Book I The main thesis is that there are "No Innate Principles." Locke wrote, "If we will attentively consider new-born children, we shall have little reason to think, that they bring many ideas into the world with them." Rather, "by degrees, afterwards, ideas come into their minds; and...they get no more, nor no other, than what experience, and the observation of things, that come in their way, furnish them with." Book I of the Essay is an attack on nativism or the doctrine of innate ideas; Locke indeed sought to rebut a prevalent view of innate ideas that was firmly held by philosophers of his time. While he allowed that some ideas are in the mind from an early age, he argued that those ideas are furnished by the senses starting in the womb—for instance, differences between colours or tastes. If we have a universal understanding of a concept like sweetness, it is not because this is an innate idea, but because we are all exposed to sweet tastes at an early age. One of Locke's