All thought or perception is, according to Leibniz, thought or perception of something. But what is really outside the monad can only be the other monads and God, and, since there is no external influence exerted on the monad, it can only ideally repre sent what is taking place in its environment. It follows that what we describe as a material world, extended in space, must be phenomenal in character, a way in which ultimate reality is obscurely represented through means of sense-perception. Never theless, owing to the singleness of plan underlying the develop ment of all the monads, it is not to be supposed that material nature has only that mode of being we assign to images of phantasy. It is a well-founded phenomenon (bene fundatum). So, too, that characteristic of the material world which the Cartesians had taken to be existent in the same sense that minds are existent, spatial extendedness, must likewise be phenomenal in character. Yet, there is a reality underlying what we perceive as spatial extendedness,—namely, the order of coexistence of the monads, no substance but a relation among substances.
The characteristics of space as apprehended are, then, in one sense subjective, in that they do not correspond to what is real, and in another sense objective in that they are common to the perceptions of all the monads at a certain stage of development. It is doubtful how far Leibniz would have been willing to follow out the line of reflection involved in the consideration that with increasing advance of knowledge the limitations of our spatial picture of things would be gradually removed. He certainly held that in the divine mind, the ideas forming the objects of the divine intellect are not contemplated as in spatial relations to one another.
It would, therefore, have been possible for Leibniz to maintain that the spatial form is incidental to that confused mode of apprehension which we call sense-perception. This is what Kant did take to be implied in his doctrine. It constitutes, indeed, one of Kant's main criticisms of the Leibnizian position that according to it the sensuous apprehension of things which clothes them with space-relations and the intellectual apprehension which leaves such space-relations out differ only in degree of clearness, that it is the same world of things confusedly apprehended by sense as a spatially extended realm which is intellectually apprehended as related only in the fashion of coexistences, whatever that phrase may mean. And, of course, similar considerations apply to Leibniz's contention that time, as we apprehend it in per ception, is but the obscure and confused picture of the grounds which determine the order of succession, whatever the term "succession" may mean.
b. Empirical Theories.—The term "experience" is an ex cessively ambiguous term, and in one or another of the various ways in which it has been employed, well-nigh every theory of knowledge might be said to be empirical. We noted, however, the
manner in which Aristotle distinguished what he called 447rEcpta from and that distinction will afford a clue to the meaning of the term in the present context. For, according to the theories now to be considered, knowledge is ultimately based upon sense-apprehension, and the objects thereby apprehended are built up of particular, isolated elements—sensations or im pressions—given to the mind from without, which in and through such apprehension are in some way welded together. So that the task of a theory of knowledge will largely consist in analysing the complex objects of perception into their simple 'constituents, and in tracing the threads of connection by which these are combined together in what is described as "experience." Locke (1632-1704) may be taken as having given the first definite exposition of an empirical theory of knowledge in the Essay concerning the Human Understanding. In this work, he endeavours to conduct the inquiry into the origin, certainty and extent of human knowledge "in a simple historical way," that is, by showing whence and how we come to have "ideas," and by examining the processes of manipulation these underzo in the mature intelligence. Defining the term "idea" as that which serves best to stand for "whatsoever is the object of the under standing when a man thinks," his preliminary task is the negative one of disposing of the view that certain of our "ideas" are "innate" possessions of the mind itself.
Viewing, then, the mind as at the start like an "empty cabinet," void of all furniture, Locke finds that the several "ideas" .which form the raw material of knowledge are supplied to it by the two avenues, sensation and reflection, or external and internal ob servation. By the former we obtain our ideas of sensible things, by the latter our ideas of the mind's own operations, such as per ceiving, thinking, doubting, willing, etc. While so far agreeing with the Cartesian thinkers as to regard the real world as being made up of two kinds of entities, material and mental, he yet took for granted, despite the Cartesian criticism, that things and minds act causally upon one another. Things produce in us ideas of sensation by "impulse" or impression, although he was far from considering that such a metaphor explains what actually happens. Furnished, thus, with simple ideas, as its data, the mind can, in virtue of the powers or faculties with which it is en dowed, manipulate that raw material in a variety of ways. It can retain the "ideas" it passively receives, combine them, put them in relation with one another, and exercise upon them the process of abstraction. Complex ideas are, in that way formed, and they bear obvious marks of the mind's operation.