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In the second book, Locke begins his positive account of how people acquire knowledge. "Let us suppose", he writes, "the mind to have no ideas in it, to be like ''white paper'' with nothing written on it. How then does it come to be written on?...To this I answer, in one word, from ''experience''". Locke's belief that all knowledge comes from sense experience is '''empiricism'''.[[CiteRef::Locke (2015b)|p. 18]] Unlike Descartes, Locke does not seriously entertain the possibility that his senses are fundamentally unreliable. He writes that, "We certainly find that pleasure or pain follows upon the application to us of certain objects whose existence we perceive (or dream we perceive!) through our senses; and this certainly is as great as we need for practical purposes, which are the only purposes we ought to have". [[CiteRef::Locke (2015d)|p. 202]] When our senses are applied to particular perceptible objects, they convey into the mind perceptions of those things. This '''sensation''' is the source of most of our ideas. We can also perceive the workings of our own mind within us, which gives us ideas of the mind's own operations such as "perception, thinking, doubting, believing, reasoning, knowing, willing, and all the different things our minds do", a process which Locke calls '''reflection'''. [[CiteRef::Locke (2015b)|p. 18]] Simple ideas produced by these processes can be grouped into complex ideas, such as those of substances and modes. '''Substances''' are independently existing things like God, angels, humans, animals, plants, and constructed things. '''Modes''' are dependently existing things like mathematical and moral ideas, which form the content of religion, politics, and culture. Note that while Locke does not believe that we are born with ideas, he believes we are born with faculties to receive and manipulate them. [[CiteRef::Uzgalis (2016)]]

As a corpuscularist, Locke took all observable bodies to be composed of invisibly small material particles called corpuscles and took direct physical contact to be the primary or sole means of communicating motion. Material bodies had certain '''primary qualities''' that are impossible to separate from them; size, shape, texture, and motion. They also had '''secondary qualities''', which the object's abilities to produce in people sensations of color, sound, taste, and smell when they interact with their primary qualities. [[CiteRef::Kochiras (2014)]] Unlike Descartes, Locke accepted the possibility that the soul might be material. In book IV of his Essay, he wrote that "anyone who will allow himself to think freely...will hardly find reason directing him firmly for or against the soul's materiality". He argued that the materiality of the soul was consistent with "the great ends of religion and morality", since God might effect the material resurrection of the dead on judgment day. [[CiteRef::Locke (2015b)|p. 205]]