David Hume published An Enquiry Concerning Human Understanding in 1748. He was a respected empiricist philosopher, meaning he believed that all thought is based upon experience. Credited with founding the rational processes of cognitive science, Hume holds a prominent place among the great philosophers. Although he rejected many of the ideas of his predecessors - including Descartes, Darwin, and Locke, - he was hugely influential in the ideas of many successive generations of philosophers. In fact, Immanuel Kant credits his pursuit of philosophy to Hume's work.
Hume wrote An Enquiry as a follow-up to his first book A Treatise of Human Nature. A Treatise was poorly received upon its publishing, much to Hume's disappointment. Consequently, in An Enquiry Hume presented the same conclusions but in more direct terms, having removed some sections and simplified his arguments. He thought perhaps people didn't appreciate his first book simply because they didn't possess the patience to read it thoroughly. Apparently he did something right because An Enquiry has been widely accepted and revered ever since it was first published.
An Enquiry is Hume's attempt to explain thought. He was an epistemologist, so he was concerned with describing what can be known and the processes by which it is known. In this book, he first dedicates a section to explaining how he defines epistemology. Then he logically orders his arguments in a sections which progress and compound to create a complete picture of human experience.