To what extent did Berkeley achieve his announced aims? The complete answer cannot be given in brief, just as Berkeley himself could not make all the grounds and implications of his philosophy clear at once. Many readers find themselves unable to refute Berkeley’s arguments, yet they remain unconvinced by them; and many professional philosophers have given long and profound attention to the problems he raises. A great merit of this book and of the Three Dialogues Between Hylas and Philonous is that Berkeley was thorough and clever in foreseeing and forestalling possible objections. Yet criticisms exist that, while insufficient to prove a diametrically opposite position such as materialism or even a more moderate realism, nevertheless show that Berkeley’s conclusions do not necessarily follow from his premises.

He was probably correct in his insistence on the dangers of abstraction, although he sometimes seems to have confused conception with visualization. Many thinkers today would agree also with his demand that terms and statements describing the physical world be defined and verified by reference to sensory experience. However, can one infer from this experience that the world is ultimately mental or immaterial in nature? Berkeley’s argument seems either to beg the question or to depend on ambiguous terms. A fair but condensed statement of it seems to be this:1. Physical objects are objects of knowledge 2. Objects of knowledge are ideas or sets of ideas 3. Ideas and sets of ideas are in the mind, or mental 4. Therefore, physical objects are in the mind, or mental

However, “objects of knowledge” is ambiguous, unless one already grants that the world is mental; in the first sentence, it means “nonmental things,” but in the second, it means “constituents of knowledge.” Of course, the constituents of knowledge are ideas by definition, but this fact does not bestow upon knowledge the power to constitute the real nature of what would not otherwise have been considered ideal or immaterial. Whether or not one perceives or conceives a “physical” object is actually irrelevant to the object itself.

Still, this criticism does not prove that physical objects are independently real or that the term “matter” has a meaning describable in terms not ultimately derived from perception. Berkeley has a strategic advantage in the fact that all people are caught in what American philosopher Ralph Barton Perry called “the egocentric predicament”: In a sense people are forever imprisoned within their own consciousness because they must always use thought as a bridge to the “outside.” However, this advantage can also be a liability, for Berkeley’s skepticism about external reality can be turned against people’s knowledge of other minds, the eternal Spirit, and even their own minds considered as substantial entities. Scottish philosopher David Hume and subsequent philosophers, for example, have not agreed that an indivisible, incorporeal self can be discovered by inward reflection. Many of Berkeley’s conclusions, such as his account of the self’s continuity by saying that the spirit always thinks, have the appearance of absurdities demanded by his premises rather than of facts verifiable by experience. Hence, “spirit” itself may turn out to be an abstraction to be relegated to the company of “matter.”

In Berkeley’s later writings, the purity of his empiricism is diluted by noticeable amounts of rationalism, and even in A Treatise Concerning the Principles of Human Knowledge , there are assumptions hardly empirical in origin or confirmation, such as his facile acceptance of the traditional attributes of God—eternal, infinite, omniscient—as obviously pertaining to that Spirit. Berkeley’s arguments for God’s existence, which are the traditional cosmological and teleological “proofs,” would have to meet the devastating criticisms...