In deriving our knowledge from two distinct sources, Sensation and Reflection, Locke is advancing a position altogether different from that of what is properly called the Sensationalist school of philosophers. Gassendi and Hobbes before him, Condillac and Helvétius after him, found the ultimate source of all our knowledge in the impressions of sense. The emphatic words of Hobbes, standing in the forefront of the Leviathan, are:—"The original of all the thoughts of men is that which we call Sense, for there is no conception in a man's mind which hath not at first, totally or by parts, been begotten upon the organs of sense." And Condillac, aiming at a theory still more simple, derives from sensations not only all our knowledge but all our faculties. "The other fountain," then, of Locke has, we must recollect, a peculiar significance as distinguishing his psychology from that of the sensationalist writers who preceded and who followed him. His theory of the origin of knowledge may fairly be called an experiential, but it cannot with any truth be called a sensationalist theory.
The rest of the Second Book of the Essay is mainly taken up with the attempt to enumerate our simple ideas of Sensation and Reflection, and to resolve into them our other ideas, however complex. To follow Locke into these details would be to re-write the Essay. I propose simply to direct the attention of the reader to a few salient points.
Of "Simple Ideas of Sensation," some "come into our minds by one Sense only." Such are the various colours, sounds, tastes, and smells, Heat and Cold, and the sensation of Resistance or Impenetrability, which Locke denominates Solidity. "The Ideas we get by more than one sense are of Space or Extension, Figure, Rest, and Motion."
The "Simple Ideas of Reflection," which the mind acquires, when "it turns its view inward upon itself, and observes its own actions about those ideas it has received from without," are mainly two, namely, Perception or Thinking, and Volition or Willing.
"There be other simple ideas, which convey themselves into the mind by all the ways of Sensation and Reflection, namely, Pleasure or Delight, Pain or Uneasiness, Power, Existence, Unity. (Bk. II., ch. vii., § 1.)
"These simple ideas, the materials of all our knowledge, are suggested and furnished to the mind only by those two ways above mentioned, namely, Sensation and Reflection. When the Understanding is once stored with these simple ideas, it has the power to repeat, compare, and unite them, even to an almost infinite variety, and so can make at pleasure new complex ideas. But it is not in the power of the most exalted Wit or enlarged Understanding, by any quickness or variety of thoughts, to invent or frame one new simple idea in the mind, not taken in by the ways before mentioned. Nor can any force of the Understanding destroy those that are there: the dominion of man, in this little world of his own understanding, being much-what the same as it is in the great world of visible things, wherein his power, however managed by art and skill, reaches no farther than to compound and divide the materials that are made to his hand, but can do nothing towards the making the least particle of new matter or destroying one atom of what is already in being. The same inability will every one find in himself who shall go about to fashion in his Understanding any simple idea not received in by his senses from external objects or by reflection from the operations of his own mind about them." (Bk. II., ch. ii., § 2.)
In the reception of these simple ideas, Locke regards the mind as merely passive. It can no more refuse to have them, alter or blot them out, than a mirror can refuse to receive, alter, or obliterate the images reflected on it. The Understanding, before the entrance of simple ideas, is like a dark room, and external and internal sensation are the windows by which light is let in. But when the light has once penetrated into this dark recess, the Understanding has an almost unlimited power of modifying and transforming it. It can create complex ideas, and that in an infinite variety, out of its simple ideas, and this it does chiefly by combining, comparing, and separating them.
"This shows man's power, and its way of operation, to be much what the same in the material and intellectual world. For the materials in both being such as he has no power over, either to make or destroy, all that man can do is either to unite them together, or to set them by one another, or wholly separate them." (Bk. II., ch. xii., § 1.)
The complex ideas are classified under three heads, modes, which may be either simple or mixed, substances, and relations. Here, however, my analysis must stop, and I must content myself with giving a few examples of the manner in which Locke attempts to resolve "complex ideas" into "simple" ones.
The idea of Infinity, to take one of his most celebrated resolutions, is merely a simple mode of Quantity, as Immensity is a simple mode of Space, and Eternity of Duration. All alike are negative ideas, arising whenever we allow the mind "an endless progression of thought," without any effort to arrest it. "How often soever" a man doubles an unit of space, be it a "mile, or diameter of the earth, or of the Orbis Magnus," or any otherwise multiplies it, "he finds that, after he has continued this doubling in his thoughts and enlarged his idea as much as he pleases, he has no more reason to stop, nor is one jot nearer the end of such addition, than he was at first setting out; the power of enlarging his idea of Space by farther additions remaining still the same, he hence takes idea of infinite space." (Bk. II., ch. xvii., § 3.)