<i><b>An Essay Concerning Human Understanding by&nbsp;John Locke</b></i>&nbsp;is a work by&nbsp;John Locke&nbsp;concerning the foundation of human knowledge and understanding. It first appeared in 1689 (although dated 1690) with the printed title&nbsp;<i><b>An Essay Concerning Human Understanding</b></i>.<br /><br />He describes the mind at birth as a blank slate (<i>tabula rasa</i>, although he did not use those actual words) filled later through&nbsp;experience. The essay was one of the principal sources of&nbsp;empiricism&nbsp;in modern philosophy, and influenced many enlightenment philosophers, such as&nbsp;David Hume&nbsp;and&nbsp;George Berkeley.<br /><br />Book I of the&nbsp;<i>Essay</i>&nbsp;is Locke&#39;s attempt to refute the&nbsp;rationalist&nbsp;notion of&nbsp;innate ideas. Book II sets out Locke&#39;s theory of ideas, including his distinction between passively acquired&nbsp;<i>simple ideas</i>&mdash;such as &quot;red,&quot; &quot;sweet,&quot; &quot;round&quot;&mdash;and actively built&nbsp;<i>complex ideas</i>, such as numbers, causes and effects, abstract ideas, ideas of substances, identity, and diversity.<br /><br />Locke also distinguishes between the truly existing&nbsp;<i>primary qualities</i>&nbsp;of bodies, like shape, motion and the arrangement of minute particles, and the&nbsp;<i>secondary qualities</i>&nbsp;that are &quot;powers to produce various sensations in us&quot; such as &quot;red&quot; and &quot;sweet.&quot; These&nbsp;<i>secondary qualities</i>, Locke claims, are dependent on the&nbsp;<i>primary qualities</i>. He also offers a theory of&nbsp;personal identity, offering a largely psychological criterion. Book III is concerned with language, and Book IV with knowledge, including&nbsp;intuition, mathematics,&nbsp;moral philosophy,&nbsp;natural philosophy&nbsp;(&quot;science&quot;),&nbsp;faith, and&nbsp;opinion.