particular things as modes of the simple idea substance. But this he does not do. Substance is an idea regarding which he was in earnest with his own fundamental theory (although perplexed about the origin of the idea of "substance in general" as well as of the ideas of "particular sorts of substances"1); and the difficulties in which his theory involved him on this head were both provocative of criticism and fruitful for the progress of thought. He admits that substance is a complex idea; that is to say, it is formed by the mind's action out of simple ideas. Now, this idea of substance marks the difference between having sensations and perceiving things. Its importance, therefore, is clear; but there is no clearness in explaining it. We are told that there is a "supposed or confused idea of substance" to which are joined (say) "the simple idea of a dull whitish colour, with certain degrees of weight, hardness, ductility and fusibility," and, as a result, we have the idea of lead." A difficulty might have been avoided if substance could have been interpreted as simply the combination by the understanding of white, hard, etc., or some similar cluster of ideas of sensation. But it was not Locke's way thus to ignore facts. He sees that something more is needed than these ideas of sensation. They are only joined to "the supposed or confused idea of substance," which is there and "always the first and chief"." He holds to it that the idea is a complex idea and so made by the mind; but he is entirely at a loss to account for the materials out of which it is made. We cannot imagine how simple ideas can subsist by themselves, and so "we accustom ourselves to suppose some substratum wherein they do subsist," and this we call substance. In one place, he even vacillates between the assertions that we have no clear idea of substance and that we have no idea of it at all3. It is "a supposition of he knows not what." This uncertainty, as will appear presently, throws its shadow over our whole knowledge of nature.

1 Cp. II, xxiii, 1-3.

2 11, xii, 6.

3 1, iii, 19.

The new way of ideas' is thus hard put to it in accounting for the universal element in knowledge; it has even greater difficulties to face in defending the reality of knowledge. And, in the latter case, the author does not see the difficulties so clearly. His view is that the simple idea is the test and standard of reality. Whatever the mind contributes to our ideas removes them further from the reality of things; in becoming general, knowledge loses touch with things. But not all simple ideas carry with them the same significance for reality. Colours, smells, tastes, sounds, and the like are simple ideas, yet nothing resembles them in the bodies themselves; but, owing to a certain bulk, figure, and motion of their insensible parts, bodies have "a power to produce those sensations in us." These, therefore, are called "secondary qualities of bodies." On the other hand, "solidity, extension, figure, motion or rest, and number" are also held by Locke to be simple ideas; and these are resemblances of qualities in body; "their patterns do really exist in the bodies themselves"; accordingly, they are "primary qualities of bodies1." In this way, by implication if not expressly, Locke severs, instead of establishing, the connection between simple ideas and reality. The only ideas which can make good their claim to be regarded as simple ideas have nothing resembling them in things. Other ideas, no doubt, are said to resemble bodily qualities (an assertion for which no proof is given and none is possible); but these ideas have only a doubtful claim to rank as simple ideas. Locke's prevailing tendency is to identify reality with the simple idea, but he sometimes comes within an ace of the opposite view that the reference to reality is the work of thought.

1 A similar distinction between qualities of body was formulated by Galileo, Hobbes, and Descartes; its origin may be traced to Democritus; and the words 'primary' and 'secondary' were occasionally used in this connection by Robert Boyle, Origine of Formes and Qualities (1666), pp. 10, 43, 100-1; cp. Tracts (1671), introduction, p. 18.

In the fourth book of his Essay, Locke proceeds to apply these results so as to determine the nature and extent of knowledge. As ideas are the sole immediate objects of the mind, knowledge can be nothing else than "the perception of the connexion of and agreement, or disagreement and repugnancy, of any of our ideas." This agreement or disagreement is said to be of four sorts: identity or diversity; relation; co-existence or necessary connection; real existence. Each of these kinds of knowledge raises its own questions; but, broadly speaking, one distinction may be taken as fundamental. In the same paragraph in which he restricts knowledge to the agreement or disagreement of our ideas, he admits one kind of knowledge which goes beyond the ideas themselves to the significance which they have for real existence. When the reference does not go beyond the ideas 'in the mind,' the problems that arise are of one order; when there is a further reference to real things, another problem arises. The preceding books have prepared the way for the solution of both sets of problems.

When ideas are together in the mind, we can discover their relations to one another; so long as they are not taken to represent archetypes outside the mind, there is no obstacle to certainty of knowledge. "All relation terminates in, and is ultimately founded on, those simple ideas we have got from sensation or reflection "1; but! general and certain truths, are only founded in the habitudes and relations of abstract ideas2." In this way Locke vindicates the certainty of mathematics: although instructive, the science is merely ideal, and its propositions do not hold of things outside the mind. He thinks also that "morality is capable of demonstration as well as mathematics." But, in spite of the entreaties of his friend Molyneux, he never set out his ethical doctrine in detail. In the second book he had reduced moral good and evil to the pleasure and pain which—as reward and punish2 Iv, xii, 7.

1 II, xxviii, 18.

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ment-come to us from some lawgiver; thus they point to a source outside the mind. But his ground for maintaining the demonstrative character of morality is that moral ideas are "mixed modes," and therefore mental products, so that their "precise real essence...may be perfectly known." He ventures upon two examples only of this demonstrative morality; and neither of them is more than verbal or gives any information about good or evil. Yet the doctrine is significant as showing the influence upon Locke of another type of thought, of which there are many traces both in the Essay and in his other works.

Xself,

The real existences to which knowledge extends are self, God, and the world of nature. Of the first we have, says Locke, an intuitive knowledge, of the second a demonstrative knowledge, of the third a sensitive knowledge. This view he proceeds to explain and defend. Locke holds that the existence of the self is known by immediate intuition. Like Descartes, he thinks that doubt on this head is excluded. But he fails to point out how self can be an idea and thus belong to the material of knowledge. An idea of self cannot come from sensation; and the simple ideas of reflection are all of mental operations, and not of the subject or agent of these operations. On the other hand, when he had occasion to discuss personal identity, he followed his new way of ideas, and made it depend on memory. His proof of the existence of God belongs to the order called by philosophers cosmological. It starts with the existence of a thinking self or mind, and argues from this position to the necessity for an intelligent first cause. Locke assumes, without question, the validity of the causal principle even beyond the range of possible experience. It was left for David Hume to take the momentous step of questioning this principle.

Regarding self and God, therefore, Locke does not show any special originality of view. It is when he faces the question of the real existence of external bodies that

his doctrine of ideas as the sole immediate object of the understanding comes into play, and casts uncertainty upon the propositions of natural science. He does not, indeed, question the transition from the presence of an idea of sensation to the existence "at that time" of a thing which causes the idea in us1. Here, he thinks, we have "an assurance that deserves the name of knowledge?," although he admits that it is "not altogether so certain as our intuitive knowledge, or the deductions of our reason employed about the clear abstract ideas of our own minds.". Knowledge of this sort is merely sensitive; it does not extend beyond "the present testimony of our senses employed about particular objects that do then affect them3." Necessary connection here is beyond our reach. Any assertion about things, except in respect of their immediate presence to the senses-all the generalisations of natural science, therefore-fall short of knowledge strictly so called. "God has set some things in broad daylight4"; but the science of nature is not one of them; there, as in many other matters, we have only "the twilight of probability"; but probability is sufficient for our purposes. This sober practical note marks the outcome of the whole enquiry: "our faculties being suited not to the full extent of being, nor to a perfect, clear, comprehensive knowledge of things free from all doubt and scruple; but to the preservation of us, in whom they are; and accommodated to the use of life5."

In his other works Locke's practical interests find ample scope; he deals with most of the questions that attracted the mind of the day, and he left upon them the mark of his thought. In Two Treatises of Government he has two purposes in view: to refute the doctrine of absolute power, as it had been put forward by Sir Robert Filmer, and to