Locke has his place among classic writers on education. In the Thoughts on Education imaginative sentiment is never allowed to weigh against utility ; information is sub ordinate to the formation of useful character; the part which habit plays in individuals is always kept in view; the dependence of intelligence and character, which it is the purpose of education to improve, upon health of body is steadily inculcated ; to make children happy in undergoing education is a favourite precept; accumulating facts without exercising thought, and without ac customing the youthful mind to look for evidence, is referred to as a cardinal vice. Wisdom more than much learning is what he requires in the teacher. The infinity of real existence, in contrast with the necessary finitude of human understanding and ex perience, is always in his thoughts. In his Conduct of the Under standing (posthumously published i 706) the pupil is invited to occupy the point at which "a full view of all that relates to a question" is to be had, and at which alone a rational discernment of truth is possible. The uneducated mass of mankind, he com plains, either "seldom reason at all," or "put passion in the place of reason," or "direct their minds only to one part of the evi dence." Hasty judgment, bias, absence of an a priori "indiffer ence" to what the evidence may in the end require us to conclude, undue regard for authority, excessive love for custom and antiq uity, indolence and sceptical despair are among the states of mind marked by him as most apt to interfere with the formation of beliefs in harmony with the Universal Reason that is active in the universe.
The well-known Essay Concerning Human Understanding em bodies Locke's philosophy. It was the first extensive attempt to estimate critically the certainty and the adequacy of human knowledge when confronted with God and the universe. Exclud ing from his enquiry "the physical consideration of the mind," he sought to make a faithful report, based on an introspective study of consciousness, as to how far a human understanding of the universe can reach.
Locke saw that the ultimate questions about our knowledge and its extent presuppose questions about ideas. Without ideas knowledge is impossible. He uses the word idea in a peculiar way —"the term which, I think, stands best for whatsoever is the object of the understanding when a man thinks" or "whatever it is which the mind can be employed about." But ideas them selves are, he reminds us, "neither true nor false, being nothing but bare appearances," phenomena as we might call them. Truth and falsehood belong only to assertions or denials concerning ideas. That none of our ideas are "innate" is contended in book i. of the Essay. This means that the human mind, before any ideas are present to it, is a tabula rasa. This famous argument has been criticized as if it was a speculative controversy between empiricism and intellectualism, and for this, Locke himself is partly to blame; the phrase tabula rasa was misleading and seems to have been used to refute the Cambridge Platonists and Lord Herbert of Cherbury. What Locke really objects to is, that any of our supposed knowledge, even of the existence of God, or fundamental principles of morality, should claim immunity from free criticism. He believed that in attacking "innate principles" he was pleading for universal reasonableness instead of blind reliance on authority, and was thus, as he says, not "pulling up the foundations of knowledge," but "laying those foundations surer." When men heard that there were propositions that could not be doubted, it was a short and easy way to assume that what are only arbitrary prejudices are "innate" certainties.
Book ii. proposes that all human ideas, even the most complex and abstract and sublime, ultimately depend upon "experience."
They come, either from the five senses or from reflective con sciousness. He proposes to show that even those concerned with the Infinite depend at last on one or other of these two sources. To prove this, our thoughts of space, time, infinity, power, sub stance, personal identity, causality and others which "seem most remote from the supposed original" are shown to depend either on perception of things external, through the five senses, or on reflection upon operations of the mind within. Reflection, "though it be not sense, as having nothing to do with external objects," is yet, he says, "very like it, and might properly enough be called internal sense." But the suggestion that "sense" might designate both the springs of experience is misleading, when we find in the sequel how much Locke tacitly credits "reflection" with. The mind, in becoming gradually stored with its "simple ideas" is able to elaborate them in numberless modes; although it cannot invent or frame any new simple idea.
Chapter 8 of book ii. on "things and their qualities" looks like an interpolation in an analysis of mere "ideas." Locke here treats simple ideas of the five senses as qualities of outward things. And the sense data are, he finds, partly (a) revelations of external things themselves in their mathematical relations, and partly (b) sensations, boundless in variety, which are somehow awakened in us through contact with things relatively to their mathematical relations. Locke calls the former sort "primary, original or essential qualities of matter," and the others "secondary or derived qualities." The primary, which are quantities rather than qualities, are inseparable from matter, and virtually identical with the ideas we have of them. On the other hand, there is nothing perceived in the mathematical relations of bodies which in the least resembles their secondary qualities. If there were no sentient beings, the secondary qualities would cease to exist, "except perhaps as unknown modes of the primary, or, if not, as something still more obscure." On the other hand, "solidity, extension, figure and motion would," he assumes, "be really in the world as they are, whether there were any sensible being to perceive them or not." Thus Locke teaches that matter is something capable of being expressed in terms of mathematical quantity, and also in terms of our own sensations. A further step was to suggest the ultimate dependence of the secondary qualities of bodies upon "the bulk, figures, number, situation and motions of the solid parts of which the bodies con sist," these mathematical or primary qualities "existing as we think of them whether or not they are perceived." Chapters 13-28 of bk. ii. concern what may be called "crucial instances" in verification of its fundamental hypothesis of the dependence of human knowledge upon the simple ideas presented in our dual experience. Space, he says, appears when we use our senses of sight and touch; succession he finds "suggested" by all the changing phenomena of sense, and by "what passes in our minds"; number is "suggested by every object of our senses, and every thought of our minds, by everything that either doth exist or can be imagined." The modifications of which these are sus ceptible he reports to be "inexhaustible and truly infinite, ex tension alone affording a boundless field to the mathematicians." But the mystery latent in our ideas of space and time is, that "something in the mind" irresistibly hinders us from allowing the possibility of any limit to either. Thus Locke seems by implica tion to acknowledge something added by the mind to the original "simple ideas" of extension and succession ; though he finds that what is added is not positively conceivable.