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 us. [Bk. IV, chap. XI, sec. 2.] Here, he thinks, we have “an assurance that deserves the name of knowledge,” [Bk. IV, chap. XI, sec. 3.] 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 them.” [Bk. IV, chap. XI, sec. 9.] 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 daylight”; [Bk. IV, chap. XII, sec. 1.] 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 life. [Bk. IV, chap. XI, sec. 8.]
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 establish a theory which would reconcile the liberty of the citizen with political order. The criticism of Filmer is complete. His theory of the absolute sovereignty of Adam, and so of kings as Adam’s heirs, has lost all interest; and Locke’s argument has been only too effective: the exhaustive reply to so absurd a thesis becomes itself wearisome. There is little direct reference to the more enduring work of Hobbes; but this work seems to have been in Locke’s mind when he argued that the doctrine of absolute monarchy leaves sovereign and subjects in the state of nature towards one another. The constructive doctrines which are elaborated in the second treatise became the basis of social and political philosophy for many generations. Labour is the origin and justification of property; contract or consent is the ground of government, and fixes its limits. Behind both doctrines lies the idea of the independence of the individual man. The state of nature knows no government; but in it, as in political society, men are subject to the moral law, which is the law of God. Men are born free and equal in rights. Whatever a man “mixes his labour with” is his to use. Or, at least, this was so in the primitive condition of human life in which there was enough for all and “the whole earth was America.” Locke sees that, when men have multiplied and land has become scarce, rules are needed beyond those which the moral law or law of nature supplies. But the origin of government is traced not to this economic necessity, but to another cause. The moral law is always valid, but it is not always kept. In the state of nature, all men equally have the right to punish transgressors: civil society originates when, for the better administration of the law, men agree to delegate this function to certain officers. Thus, government is instituted by a “social contract”; its powers are limited, and they involve reciprocal obligations; moreover, they can be modified or rescinded by the authority which conferred them. Locke’s theory is thus no more historical than the absolutism of Hobbes. It is a rendering of the facts of constitutional government in terms of thought, and it served its purpose as a justification of the revolution settlement in accordance with the ideas of the time.