Great as was Locke's services to his country, and to the cause of civil and religious liberty, his fame rests on the Essay On the Understanding, which marks an epoch in the history of philosophy. His purpose was to inguire into the powers of the human under standing, with a view to find out what things it was fitted to grapple vvith, and where it must fail, so as to make the mind of man " more cautious in meddling with things exceeding its comprehension, and disposed to stop when it is at the utmost extent of its tether." This purpose led him to that thorough investigation of the constitution of the human mind, resulting in the most numerous and important contributions ever made by one man to our knoxviedge on this subject. He institutes a preliminary inquiry, in the subject of the first book, as to the existence of innate ideas, theoretical and practical, on which the philosophical world has been so much divided. See COMMON SENSE. Locke argues against the existence of these supposed innate conceptions, or intuitions, of the mind with a force and cogency that appear irresistible. Having thus repudiated the
instinctive sources of our knowledge or ideas, lie is bound to show how we come by them in the course of our experience. Our experience being twofold, external and internal, we have two classes of ideas—those of sensation and those of reflection. He has therefore to trace all the recognized conceptions of the mind to one or other of these sources. Many of our notions are obviously derived from experience, as colors, sounds, etc.; but some have been disputed, more especially such as space, time, infinity, power, substance, cause, mere good and evil; and Locke discusses these at length, by way of tracing them to the same origin. This is the subject of book second, entitled " Of Ideas." Book third is on language considered as an instrument of truth, and contains much valuable material. The fourth book is on the nature, limits, and reality of our knowledge, including the nature of demonstrative truth, the existence of a God, the provinces of faith and reason, and the nature of error.