A rapid analysis of this Essay is necessary to enable us to form a right estimate of the philosophical merits of Locke.
As all human knowledge ultimately reposes, both in legitimacy and extent, on the raugo and correctness of the cognitive faculty, which Locke designates by the term ' understanding,' Locke proposes to determine what objects our understanding is and is not fitted to deal with. With this view ho proposes in the first place to inquire into the origin of ideas; in the next place, to show the nature of that knowledge which is acquired by those ideas, and its certainty, evidence, and extent ; and lastly, to determine the nature and grounds of assent or opinion.
Before entering upon this investigation Locke gets rid of a sup position which, if once admitted, would render all such inquiry useless. The refutation of the theory of innate ideas and principles of know ledge is the subject-matter of the first book of the Essay. Generally, he observes, the common assent of men to certain fundamental prin ciples may be explained otherwise than by the supposition of their being innate ; and consequently the hypothesis is unnecessary. But, in particular, ho denies that there are any such universal and primary principles as are admitted by all men, and known as soon as developed, for to these two heads be reduces all the arguments usually advanced in support of this hypothesis. Thus of speculative principles ho takes the principles of contradiction and identity, and shows, by an inductive appeal to savages, infants, and idiots, that they are not universally acknowledged; and as to their being primary, he appeals to observation of the infant mind, as proving that they are far from being the first ideas of which the human mind is conscious. The principles of morals are next submitted to a similar examination; and lastly, he shows that no ideas are iunato ; for this purpose he selects the ideas of God and substance, whieh, by a like appeal to savage nations and children, he proves to be neither universal nor primary, and arrives at the conclusion that neither particular ideas nor general principles of knowledge or morals are antecedent to experience.
The only source of human knowledge is experience, which is two fold, either internal or external, according as it is employed about sensible objects or the operations of our minds. Hence there are two kinds of ideas, ideas of sensation and ideas of reflection. Reflec tion might properly be called an internal sense. The latter are subsequent to the former, and are inferior in distinctness to those furnished to the mind through the sensuous impressions of outward objects. Without consciousness it is, according to Locke, impossible to have an idea ; for to have an idea and to be conscious of it is the same thing. He accordingly maintains, at great length, against Descartes, that the mind does not always think, and that its essence does not consist in thinking.
Now all ideas, whether of sensation or reflection, correspond to their objects, and there is no knowledge of things possible except as determined by our ideas. These ideas are either simple, and not
admitting of further reduction, or complex. The simple rise from the inner or outer sense ; and they are ultimately the sole materials of all knowledge, for all complex ideas may be resolved into them. The understanding cannot originate any simple ideas, or change them, but must passively receive them as they are presented to it. Locke here makes the first attempt to give an analysis of the sensuous faculty, to refer to each of the senses the ideas derived from them separately, or from the combined operation of several. Thus light and colour are derived from vision alone, but extension and figure from the joint action of sight and touch. While the outer sense gives the ideas of solidity, space, extension, figure, motion, and rest, and those of thought and will are furnished by the inner sense or reflec tion, it is by the combined operation of both that we acquire the ideas of existence, unity, power, and the like. In reference to the agree meot of ideas with their objects, Locke draws an important distinc tion between primary and secondary qualities : the former belong really to objects, and are inseparable from them, and are extension, solidity, figure, and motion ; the latter, which are colour, smell, sounds, and tastes, cannot be considered as real qualities of objects, but still, as they are powers in objects themselves to produce various sensations in the mind, their reality must in so far ho admitted. Of the operations of the understanding upon its ideas, perception and retention are passive, but discerning is active. By perception Locke understands the consciousness or the faculty of perceiving whatever takes place within the mind; it is the inlet of knowledge, while reten tion is the general power by which ideas once received are preserved. This faculty acts either by keeping the ideas brought into it for some time actually in view, which is called contemplation or attention, the pleasure or pain by which certain ideas are impressed on the senses contributing to fix them in the mind; or else by repetition, when the mind exerts a power to revive ideas which after being imprinted have disappeared. This is memory, which is, as it were, the storehouse of ideas. The ideas thus often 'refreshed,' or repeated, fix themselves most clearly and lastingly in the mind. But in memory the mind is oftentimes more than barely passive, the re-appearance of obliterated pictures or ideas depending on the will. Discerning, by which term he designates the logical activity of the intellect, consists in comparing and compounding certain simple ideas, or in conceiving them apart from certain relations of time and place. This is called abstraction, by means of which particular Ideas are advanced to generals. By composition the mind forms a multitude of complex ideas, which are either modes, substances, or relations.