The new principle which Berkeley claimed to have discovered may be expressed as follows. A unit or moment of experience is always a mind or conscious subject apprehending an object. Berkeley certainly did not mean that the object and the act of apprehending are identical, but only that they are inseparably connected, and that if they be thought of as existing independ ently, experience is being treated abstractly. To indicate its de pendence on mind, he called the object, in accordance with Locke's nomenclature, an "idea"; and the emphasis laid upon this de pendence probably suggests that the existence of the object is secondary and derivative.
From a certain point of view, Berkeley no doubt would have said that in the synthesis of subject and object, the former is primary both in the order of thought and in that of fact. But he who is serious with the notion of the correlation of mind and object must regard the correlation as holding equally of each of the terms. If there can be no objects or ideas without a mind, neither can there be a mind without objects or ideas. And, as a matter of fact, such mutual dependence is explicitly admitted by Berkeley. In truth, then, using the phrase "idea of" is one great cause of mistake ; ideas are not intermediaries between the mind and its objects, they are its objects. Yet nature, the course of external events, seems to have an externality and independence of its own, and these features have an immense significance in all our practical experience. They require, therefore, to find ex planation in terms of a theory according to which mind and ideas are the sole components of experience.
In proceeding to face this problem, Berkeley points first of all to the distinction that must be drawn between ideas of imagina tion and memory which arise through our own productive agency, and ideas of perception which come to us with such qualities as cannot be ascribed by us to our own agency, and in an order in dependent of our will. Berkeley, then, assumes that these given ideas must have a cause. And since the notion of cause can find no expression at all in terms of objective experience, the causal agency must be mind. Since, further, no more than an excessively small proportion of our ideas can be ascribed to other finite minds, there remains no alternative but to have recourse to the hypothesis of an infinite mind.
On the one hand therefore, considered objectively, the world of perception is independent of finite minds, and has its own unity and identity, because it is constituted by the way in which one infinite mind consistently and systematically affects our finite minds, and indeed there is a permanent support for ideas in that infinite mind. On the other hand, subjectively considered, as regards our belief in the externality and independence of what we perceive, the explanation is to be found in the fact that owing to the regular order and combination in which sense-ideas are given to us, they tend to be associated, so that each becomes in our experience a sign which suggests the ideas habitually conjoined with it.
There are manifest, however, in Berkeley's procedure as in Locke's, two lines of thought which refuse to be brought together. On the one hand, he preserves a large portion of the empirical theory of knowledge ; on the other hand there is involved in his speculation a conception of mind and of our ways of knowing mind that cannot even be stated in terms of the empirical theory. For he was compelled to admit that minds are not to be known through means of "ideas"; neither of our own mind nor of other minds have we "ideas" but only "notions," although how such "notions" differ from "ideas" and how the difference is com patible with his analysis of experience he nowhere attempts to determine. A similar difficulty comes to the front in his attempt to account for the generality of mathematical knowledge. More over, it can scarcely be doubted that the conception of a world of ideas in the mind of God is exposed to the very criticism which Berkeley directed upon the notion of material substances. In fact, there are numerous indications, especially in his later writings, that Berkeley's reflection was tending rather to the later Critical theory than to the more strenuous empirical doctrine of his immediate successor.