An Essay Concerning Human Understanding By John Locke
"No man's knowledge here can go beyond his experience." - John Locke
In An Essay Concerning Human Understanding, John Locke explores the concepts of how we think and perceive the world around us. After 20 years of tweaking and perfecting, the final product consists of four different books. In Book I, Locke writes about how no human is born with any innate knowledge. Locke points out that if there were any innate principles, everyone would confirm them, and there are no principles that everyone agrees on. This is an attempt to disprove the Cartesian idea of knowledge. Locke goes on to use various examples to emphasize the point that we are born with a "clean slate", onto which ideas,concepts, and knowledge must be inscribed. He goes on to argue that there is no innate knowledge of god because there is no universal consent on this "moral knowledge". Locke believes that all knowledge is gained through experience, questioning and reasoning.

In Book II, Locke expresses his theory of ideas. He attempts to locate the source of all our ideas, which either come through our senses or the mind's reflection. Locke believes that knowledge must be gained in the form of ideas. He distinguishes between primary and secondary qualities and, specifically, discusses the relationship between the qualities of the outside world and our ideas about those qualities. Another main idea in Book II is the identity of a person and its relationship to a person's "consciousness". He argues that the identity of a person rests entirely in
consciousness. He expresses his belief that there is nothing like innate knowledge.

In Book III, "Of Words," Locke focuses more on the philosophy of language, as compared to his main theme of ideas. He, nevertheless, does not undermine or ignore the philosophy of ideas. In accordance with what Locke speaks of in his third book, “words” represent “ideas” in our head. Here Locke reiterates his books’ underlying theme of ideas. On the basis of the aforesaid theories, Locke explains how we form words and use language on basis of our environment and surroundings, and the objects that comprise it. He ends the book by analyzing the weaknesses of language and the “linguistic imperfection” related with our speech.

In Book IV, "Of Knowledge and Opinion," Locke concentrates his attention to the philosophy of knowledge. He starts the book by defining knowledge, which many believe is impossible. He, however, gives the reader a stringent outline to what the meaning of the word is. Knowledge, according to Locke, “is the perception of strong internal relations that hold among the ideas themselves, without any reference to the external world. “ He then delves into his theories of relations between ideas that comprise knowledge, for instance, diversity and coexistence. Towards the end of the book, Locke gives us a account of the “three-grades” of knowledge. In other words, he speaks of the three branches of knowledge. He ends the book with the chapter “Judgment or Opinion”, which, as the name suggests, discusses identifying truth or false. He lists four sorts of relations between ideas that would count as knowledge (identity/diversity, relation, coexistence, actual existence), and then distinguishes between three grades of knowledge (intuition as the highest, demonstration as a middling level, and sensitive knowledge as a sort of pseudo- knowledge). The remainder of the book is spent discussing opinion or belief, which is the best we can hope for from nearly all our intellectual endeavors.

Book 1: Of Innate Notions
Book 2: Of Ideas
Book 3: Of Words
Book 4: Of Knowledge and Opinion

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