Berkeley's argument for Idealism is relatively straightforward and concisely presented in the first few sections of the Principles. His starting point is the empiricist assumption that the objects of knowledge are ultimately ideas. This is a technical term in the lexicon of the empiricists, but it's fairly easy to demonstrate what Berkeley means by it from the catalog he gives in the first paragraph of the Principles. Ideas come in three varieties. They might be (1) immediate sense data or perceptions, (2) ideas derived from reflection on the workings of our own minds, or (3) ideas that derive from memory or imagination. Ideas can get combined in various ways, and our object concepts are bundles of individual ideas that we find aggregate together consistently. For example, our idea of an apple is merely the set of properties we perceive clustered together during our various apple perception experiences:
Thus, for example, a certain colour, taste, smell, figure, and consistency having been observed to go together, are accounted one distinct thing, signified by the name apple (P 1).
In addition to the ideas that serve as objects of knowledge, Berkeley thinks there is obviously something that is able to perceive, remember, combine, and know these objects of knowledge. This 'something' Berkeley takes to be the mind (also known as soul or self or spirit). This mind is not some individual idea, but is a distinct subject in which ideas exist, which is to say in which they exist as perceived. Ideas, as ideas, require a perceiver for their existence (P 2). Now, there's clearly a lot of smuggled metaphysics in this notion, but none of it would have been controversial to Berkeley's empiricist audience (although on some interpretations Hume would dispute how certain we really can be about a self independent of our perceptions). And it seems plausible enough to us, as well. We have perceptions, and we're something different from perceptions.
So far, so good. Perceptions exist; perceivers exist. Now, what about objects? Here's where it gets fun. What is it that we mean when we say an object such as the table in my study exists? Berkeley's answer to this question is pivotal to his whole argument, so I'll let him speak for himself:
So far, so good. Perceptions exist; perceivers exist. Now, what about objects? Here's where it gets fun. What is it that we mean when we say an object such as the table in my study exists? Berkeley's answer to this question is pivotal to his whole argument, so I'll let him speak for himself:
The table I write on, I say, exists, that is, I see and feel it; and if I were out of my study I should say it existed, meaning that if I was in my study I might perceive it, or that some other spirit actually does perceive it (P 3).
Berkeley thus cashes out the concept of the existence of an object in terms of its being perceptible. This might seem a little strange, but it isn't a crazy way to proceed, especially for an empiricist. Even in modern science and philosophy we tend to carve out the distinction between what's real and what's not in terms of what experimental methods can tell us--in other words, what we can detect, either with normal perception or advanced instruments. Roughly put, if you can't measure--at least in principle--some proposed entity, either directly or indirectly through its effects on other things, then you're going to have a hard time convincing anyone it's a real thing.
The next move in the argument goes from this recognition that the objects of our knowledge exist as perceived ideas in the mind to the claim that their existence is just that existence as an idea being perceived: in Berkeley's famous locution, their "esse is percipi." In other words, it is not only the case that all we know are ideas in the mind, and not external objects, but those perceived ideas are all there really is. Despite the obvious craziness of this idea, Berkeley feigns disbelief that it isn't common sense:
What does all of this have to do with the reality of objects independent of our minds? Berkeley thinks that the belief that our ideas can be abstract and general has led people to believe that ideas can exist independently of perception, and that in turn causes us to make the mistake of believing that the objects of our perception (i.e., sensible objects of knowledge) can exist independently of perception. To Berkeley the possibility is simply incoherent:
Now, all these seems immediately and intuitively implausible, but Berkeley's theory is difficult to dispute from within, and marshaling external resources in a way that isn't question begging is more difficult than you'd think at first glance.
One immediate criticism that comes to mind is that Berkeley has simply made a category mistake. We might agree with him about the nature of our sense data and its dependence on the mind, but think that there is still a divide between those things out in the world that produce our ideas and the ideas themselves. In other words, Berkeley seems to have drawn ontological conclusions from epistemic considerations in an illicit way. The problem with this line of critique is that it either simply begs the question against Berkeley or is indemonstrable in Berkeley's terms. You can't simply assert that there is an external world, that's what Berkeley has attempted to disprove. But worse, you can't demonstrate it. How is it that we demonstrate the existence of things? Well, by pointing out that they are perceptible. But if they're perceptible, that means that they are possible objects of knowledge and thus--you guessed it--ideas in the mind, and therefore subject to the preceding arguments.
In the Three Dialogues, poor, slow-witted Hylas (whose name is derived from the Greek word for matter, hyle) tries to posit an unknown, ultimately unknowable "matter" discoverable by reason rather than sense in to save the ability to distinguish between world and mind. But his interlocutor Philonous ("lover of mind"), having earlier gotten Hylas to assent to the proposition that all sensible qualities are in the mind, gradually gets Hylas to admit that all those properties by which matter normally gets defined--"extended, solid, movable, unthinking, inactive substance" (D 164)--are merely sensible qualities, which means they are present only in the mind. So moving beyond them to some undetectable material substrate is nothing but an unsupported hypothesis.
Despite the fact that Berkeley's ideas can't be easily refuted by the most obvious objections, there are still some good philosophical reasons for rejecting his approach (in addition to the common sense reason that it's batshit crazy).
One consequence of Berkeley's theory is that all the various perceptions we have of an everyday object are in fact different objects of experience (D 191-2). The visual perception of my dining room table seen from the doorway is a different object than the visual perception of my view of the table from my chair. And both of these are different objects than the feel of the table if I bump into it in the dark or saw off a chunk and look at it under a microscope. My concept of my table as a single thing is a construction of my own mind, not something I receive from the outside.
But there are serious problems with this. First, it's not obvious how Berkeley can distinguish between fantasy and reality. It's clear that normal life differs from imagination in its coherence and consistency, something that makes little sense if both phenomena are merely ideas in the mind, neither connected to anything else. But a deeper problem lurks as well; Berkeley has to account for the actual stability of the world of sense when there are seemingly no constraints. But the laws of physics obtain. Our observations of the world allow us to understand how different views of the same object cohere. For example, we can make predictions about the how a large chunk of mineral viewed by the naked eye will behave by learning about its molecular structure under a microscope. This makes sense if the mineral is the same object viewed in two ways, but begs for an explanation on Berkeley's view.
One possible avenue for answering the first problem that is available to other philosophical views is that other people can also see things that are real, and their perceptions can be used to regulate our interpretation of our own. If I think I see a bear in my yard I can ask my wife to look and see if there is a bear. If she sees it too, then we probably need to call animal control. But this way around the problem isn't open for Berkeley, for obvious reasons. If the objects of perceptions are only in the mind, it's clear that two people can't ever see the same thing. Berkeley understates the severity of this problem; he dismisses it as an unimportant matter of semantics as to whether two people see the "same" object or not (D 193). But knowing whether we share a common world with others is more than mere semantics; indeed, it's hard to see how Berkeley can avoid his view collapsing into solipsism.
Berkeley does have an argument to attempt to avoid solipsism by proving the existence of both other minds and God. The only way we can know that others exist, he claims, is by the collection of ideas they excite within us. In other words, we have no immediate perception of other agents but rather infer their existence from their effects on us. So we can be confident other persons ('spirits' in Berkeley's parlance) exist. Similarly, we can be sure God exists because of his effects. Those regularities in nature mentioned above as crying out for explanation are the evidence of God's existence. Berkeley claims (quoting St. Paul) that there is some "Spirit, 'who works in all', and 'by whom all things consist.' (P 145-6).
But surely this can't work--the inferential move by which we come to certainty about the existence of spirits and God is exactly the same inferential move by which common sense tells us that external objects and matter exist, which Berkeley has claimed illegitimate. Ultimately, to maintain its coherence and to avoid solipsism and the collapse of the distinction between reality and imagination, Berkeley's whole project depends on accepting a line of reasoning it previously rejected. But if that mode of inference is legitimate after all, there is nothing left to generate the scepticism about the external world that motivated the idealist response in the first place.
The next move in the argument goes from this recognition that the objects of our knowledge exist as perceived ideas in the mind to the claim that their existence is just that existence as an idea being perceived: in Berkeley's famous locution, their "esse is percipi." In other words, it is not only the case that all we know are ideas in the mind, and not external objects, but those perceived ideas are all there really is. Despite the obvious craziness of this idea, Berkeley feigns disbelief that it isn't common sense:
It is indeed an opinion strangely prevailing amongst men, that houses, mountains, rivers, and in a word all sensible objects have an existence natural or real, distinct from their being perceived by the understanding.... For what are the forementioned objects but the things we perceive by sense, and what do we perceive besides our own ideas or sensations; and is it not plainly repugnant that any one of these or any combination of them should exist unperceived (P 4).Berkeley thinks the culprit preventing our ability to recognize the true existential basis of objects in perception is our continued belief that abstract ideas are possible (P 5). The introduction of the Principles is an extended critique of the concept of abstract ideas, in which he argues that abstract ideas (i.e., general ideas that stand for a number of particulars but lack any particular characters) are actually impossible. The point is plausible. For example: form a mental representation of the concept "triangle." What you have in your mind is some triangle, maybe it's acute, maybe it's equilateral, maybe it's obtuse. But it certainly has some particular arrangement of its angles and sides. The fact that we can think about the relations of parts of triangles independently of the particular mental image of triangle we have in mind doesn't mean our idea of triangle is a general one--only that not every feature of a triangle is relevant to a demonstration regarding triangles, so your mental equilateral triangle can serve in your conceptualizing during a demonstration that the interior angles sum to 180 degrees just as well as my mental image of a rectilinear triangle.
What does all of this have to do with the reality of objects independent of our minds? Berkeley thinks that the belief that our ideas can be abstract and general has led people to believe that ideas can exist independently of perception, and that in turn causes us to make the mistake of believing that the objects of our perception (i.e., sensible objects of knowledge) can exist independently of perception. To Berkeley the possibility is simply incoherent:
[A]s it is impossible for me to see or feel anything without an actual sensation of that thing, so is it impossible for me to conceive in my thoughts any sensible thing or object distinct from the sensation or perception of it (P 5).The consequences of all of this are, for Berkeley, straightforward. The objects of our experience are merely ideas in the mind and dependent upon the mind for their existence. If there is no mind currently perceiving some object, then that object does not exist; if the object is not being perceived by some finite mind, then it must be perceived by an eternal mind to be preserved as a real thing (P 6). The ultimate constituents of nature are not material entities like atoms and quarks and things like that, but rather, minds. The first sentence of Principles 7 provides the upshot of the whole argument:
From what has been said, it follows, that there is not any other than spirit, or that which perceives.And that's it.
Now, all these seems immediately and intuitively implausible, but Berkeley's theory is difficult to dispute from within, and marshaling external resources in a way that isn't question begging is more difficult than you'd think at first glance.
One immediate criticism that comes to mind is that Berkeley has simply made a category mistake. We might agree with him about the nature of our sense data and its dependence on the mind, but think that there is still a divide between those things out in the world that produce our ideas and the ideas themselves. In other words, Berkeley seems to have drawn ontological conclusions from epistemic considerations in an illicit way. The problem with this line of critique is that it either simply begs the question against Berkeley or is indemonstrable in Berkeley's terms. You can't simply assert that there is an external world, that's what Berkeley has attempted to disprove. But worse, you can't demonstrate it. How is it that we demonstrate the existence of things? Well, by pointing out that they are perceptible. But if they're perceptible, that means that they are possible objects of knowledge and thus--you guessed it--ideas in the mind, and therefore subject to the preceding arguments.
In the Three Dialogues, poor, slow-witted Hylas (whose name is derived from the Greek word for matter, hyle) tries to posit an unknown, ultimately unknowable "matter" discoverable by reason rather than sense in to save the ability to distinguish between world and mind. But his interlocutor Philonous ("lover of mind"), having earlier gotten Hylas to assent to the proposition that all sensible qualities are in the mind, gradually gets Hylas to admit that all those properties by which matter normally gets defined--"extended, solid, movable, unthinking, inactive substance" (D 164)--are merely sensible qualities, which means they are present only in the mind. So moving beyond them to some undetectable material substrate is nothing but an unsupported hypothesis.
Despite the fact that Berkeley's ideas can't be easily refuted by the most obvious objections, there are still some good philosophical reasons for rejecting his approach (in addition to the common sense reason that it's batshit crazy).
One consequence of Berkeley's theory is that all the various perceptions we have of an everyday object are in fact different objects of experience (D 191-2). The visual perception of my dining room table seen from the doorway is a different object than the visual perception of my view of the table from my chair. And both of these are different objects than the feel of the table if I bump into it in the dark or saw off a chunk and look at it under a microscope. My concept of my table as a single thing is a construction of my own mind, not something I receive from the outside.
But there are serious problems with this. First, it's not obvious how Berkeley can distinguish between fantasy and reality. It's clear that normal life differs from imagination in its coherence and consistency, something that makes little sense if both phenomena are merely ideas in the mind, neither connected to anything else. But a deeper problem lurks as well; Berkeley has to account for the actual stability of the world of sense when there are seemingly no constraints. But the laws of physics obtain. Our observations of the world allow us to understand how different views of the same object cohere. For example, we can make predictions about the how a large chunk of mineral viewed by the naked eye will behave by learning about its molecular structure under a microscope. This makes sense if the mineral is the same object viewed in two ways, but begs for an explanation on Berkeley's view.
One possible avenue for answering the first problem that is available to other philosophical views is that other people can also see things that are real, and their perceptions can be used to regulate our interpretation of our own. If I think I see a bear in my yard I can ask my wife to look and see if there is a bear. If she sees it too, then we probably need to call animal control. But this way around the problem isn't open for Berkeley, for obvious reasons. If the objects of perceptions are only in the mind, it's clear that two people can't ever see the same thing. Berkeley understates the severity of this problem; he dismisses it as an unimportant matter of semantics as to whether two people see the "same" object or not (D 193). But knowing whether we share a common world with others is more than mere semantics; indeed, it's hard to see how Berkeley can avoid his view collapsing into solipsism.
Berkeley does have an argument to attempt to avoid solipsism by proving the existence of both other minds and God. The only way we can know that others exist, he claims, is by the collection of ideas they excite within us. In other words, we have no immediate perception of other agents but rather infer their existence from their effects on us. So we can be confident other persons ('spirits' in Berkeley's parlance) exist. Similarly, we can be sure God exists because of his effects. Those regularities in nature mentioned above as crying out for explanation are the evidence of God's existence. Berkeley claims (quoting St. Paul) that there is some "Spirit, 'who works in all', and 'by whom all things consist.' (P 145-6).
But surely this can't work--the inferential move by which we come to certainty about the existence of spirits and God is exactly the same inferential move by which common sense tells us that external objects and matter exist, which Berkeley has claimed illegitimate. Ultimately, to maintain its coherence and to avoid solipsism and the collapse of the distinction between reality and imagination, Berkeley's whole project depends on accepting a line of reasoning it previously rejected. But if that mode of inference is legitimate after all, there is nothing left to generate the scepticism about the external world that motivated the idealist response in the first place.
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