This is the third book of John Locke's Essay on Human Understanding. Book I was Neither Principles Nor Ideas Are Innate. Book II was Of Ideas and Book III is Of Words. Locke is writing about the ideas we have in our minds and the things they are to represent. What does it mean to define a thing? What is an abstract idea? What is motion? What is essential to me as an individual? What would an Englishman born in Jamaica think about ice when he went to England for the first time? What is a concrete term? Locke tackles many such questions in this third book. Book IV is Of Knowledge and Probability. - Summary by Craig Campbell