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Berkeley's Idealism

(from my book The Great Philosophers)

‘To be is to be perceived.’

Just as younger children sometimes wonder whether the refrigerator light stays on once the door is shut, so the more  philosophically minded older child may question whether  physical objects continue to exist when they are not observed. George Berkeley’s answer to this question is that they do not. According to Berkeley, the physical world exists only while it is being perceived. So what led him to this astonishing conclusion?

Berkeley had two overriding philosophical concerns. The first was to deal with sceptical worries about the material world.  How, can we know that such a world exists? The second was to counter what Berkeley saw as the growing tendency of the scientists  and philosophers of his day to push God to the periphery in their thinking about the world. Scientists were beginning to adopt an increasingly mechanistic view of how the universe worked, with God required, at best, merely to crank the starting handle on the great world-machine, after which his presence was no longer required. Berkeley wanted to bring God back to centre stage.


Let’s begin with the threat of scepticism. According to the representational theory of perception embraced by many of the leading thinkers of Berkeley’s day, we do not perceive the world directly. Rather, our perception of the world is mediated by certain mental entities called ideas.

Suppose, for example, that you look at a tomato on the table in front of you. When you observe the tomato, what you are immediately aware of is not the tomato itself, but certain sensory appearances that parade, as it were, before your mind’s eye. What you experience directly are shifting ideas of shape, colour and so on, sliding across your internal, subjective cinema screen. The tomato itself lies behind these sensory appearances as their cause.

Berkeley’s concern about this representational theory of perception (which heassociates in particular with the philosopher John Locke, although it is debatable whether Locke endorses it) is the difficulty of knowing whether our senses are a reliable guide to external, physical reality. If we never get to experience that reality directly, to check that there is anything out there corresponding to our ideas, what grounds have we for supposing such a reality exists? Rather than mediating perception of physical reality, ideas seem to form an impenetrable veil – a barrier beyond which we can never peek – and so threaten to cut us off from knowledge of the world. Philosophers call this the veil-of-perception problem.

Berkeley’s solution to the veil-of-perception problem is ingenious. Rather than supposing that physical objects lie behind our sensory experiences, why not just suppose that they are those sensory experiences? When you observe a tomato, the tomato is not the cause of your ideas. Rather, it just is those ideas. As there is no particular problem explaining your knowledge of your own ideas, so the sceptical problem generated by the representational theory of perception is immediately solved.

If it isn’t observed, it isn’t there

Of course, while this move might indeed deal with the veil-of perception problem, it has some very odd consequences. For a start, ideas are mental entities. They are subjective in the same way that, say, pains are subjective. Just as there could not be a  pain that no one felt, so there could not be an idea which no one experienced.

It follows then that, if physical objects are just ideas or collections of ideas, they too are mental entities incapable of existing independently of being experienced.

Berkeley’s idealism has the bizarre-sounding consequence that, if no one is experiencing that tomato, there is no tomato. Those portions of the physical that are not observed do not exist. According to Berkeley, for the physical world to be is to be perceived.

The rejection of materialism

Berkeley’s idealism, which simply identifies physical objects with ideas, involves the rejection of the materialist philosophy that says that physical objects are material substances in their own right capable of mind-independent existence. The only genuine substances, according to Berkeley, are mental substances – minds. Berkeley does not deny that physical objects exist, but he maintains that they are not anything over and above the ideas entertained by minds. There are no material substances, only mental substances.

The role of God

Berkeley’s idealism may deal with a sceptical worry generated by the representative theory of perception, but what of Berkeley’s other concern – to bring God back centre stage? How does Berkeley’s idealism achieve that?

Actually, Berkeley does not deny that physical objects continue to exist when we are not perceiving them – that tomato remains on the table even while none of us observes it; your kitchen continues to exist even after you have turned off the light and gone to bed. Why? Because God constantly observes everything. And so, while the materialist philosophers of the day were finding less and less use for God in their thinking about the physical universe, Berkeley gives God a central, universe-sustaining role. The universe is kept in existence, while we do not observe it, by God’s constant gaze.

Why the tree continues to be…

The role that Berkeley’s Idealism assigns to God is nicely summarized in a limerick penned (at least in part) by Monsignor Ronald Knox:

There was a young man who said, ‘God
Must think it exceedingly odd
If he finds that this tree
Continues to be
When there's no one about in the Quad.’


Dear Sir:
Your astonishment's odd:
I am always about in the Quad.
And that's why the tree
Will continue to be,
Since observed by
Yours faithfully,

Berkeley’s master argument

We have outlined Berkeley’s extraordinary theory, but why should we accept it? What grounds do we have for supposing that it is true? In particular, why should we accept that physical objects cannot exist unperceived?

Berkeley offers a number of arguments for this conclusion, but one in particularstands out. So confident is Berkeley in this particular argument that he is prepared to let everything rest on it. It is, for this reason, often referred to as Berkeley’s master

Berkeley simply challenges us to try to conceive of a physical object that exists unperceived. Try, for example, to imagine a tree that continues to exist though no one observes it. Can you do this?

No, says Berkeley. You can’t. For in imagining the tree, you still imagine yourself perceiving it. You imagine yourself looking at it.

Berkeley concedes that while it might seem as if we can entertain the thought that there is a world of unperceived and unconsidered physical objects, it turns out, on closer inspection, that we can’t.


Is Berkeley’s master argument cogent? An initial worry we might raise is that it appears to take for granted a rather imagistic view of thought. It seems that, in Berkeley’s view, to think about something is to entertain some sort of mental image or other sensory representation of it. When I think of a tree, I conjure up a mental image of a tree, but then I do, after all, imagine myself looking at it.

However, is this way of thinking about thinking correct? Not obviously. Clearly, I can conjure up a mental image of a tree.

However, must entertaining the thought that there exists an unperceived tree involve any such an image?

It seems not. Suppose you ask me to visualize a tree. I do so. If you then ask me to describe my visualized tree, I will be able to do so. For in visualizing a tree, I inevitably imagine it having various features that I can then go on to tell you about. For example, my tree may be deciduous or coniferous, rounded or tall, with leaves or without.

If, on the other hand, you ask me simply to suppose there is a tree that exists unperceived, and then ask me to describe it, it may well turn out that I don’t have in mind any particular sort of tree at all. The tree in question need be neither deciduous
nor coniferous, neither short nor tall, neither with leaves nor without.

This rather tells against the assumption that thinking of something involves conjuring up a mental image of it. But, if we can think of something without thinking of ourselves as perceiving it in some way, doesn’t Berkeley’s argument therefore

Perhaps not. The reply to this objection may be that, even if you can suppose there exists an unperceived tree, you certainly can’t suppose that there exists one that no one thinks about. Any tree you think of will inevitably be a tree that someone is
thinking about – namely, you.

In other words, to suppose that you can conceive of a tree no one conceives of involves, as Berkeley himself points out, a contradiction – the tree in question would have to be both conceived and unconceived, both thought of and not thought  of – which is an impossibility.

So perhaps Berkeley can at least show that you are unable to entertain the thought that there exists a tree that exists unconsidered by anyone.


Unfortunately, the above argument is also fallacious. We can and should distinguish between conceiving of a particular so and so, and conceiving that there is a so and so. I can, for example, conceive that there was a US president who wore purple underpants without conceiving of any particular US president (e.g. Lincoln or Reagan) wearing purple underpants. There need be no particular person of whom I am thinking when I suppose that there is such a person.

Armed with this distinction, we can now see why Berkeley’s argument fails. To suppose we can conceive of something of which no one conceives involves a contradiction. But there is no such contradiction involved in supposing we can conceive that there is something of which no one conceives. For that is not yet
to conceive of anything at all.

So Berkeley’s conclusion doesn’t follow. Berkeley has not shown we can’t think that there exist things not thought of by any mind.

Illusions and hallucinations

Berkeley’s idealism faces a famous difficulty: how to account for hallucinations and other perceptual illusions.

Suppose that, while ill and delirious, I begin to hallucinate pink elephants dancing round my lampshade. Now the way in which we would ordinarily explain this discrepancy between how things look and how things really are is by saying that the
elephants exist only in my mind. There is nothing corresponding to my experience in external, physical reality.

This explanation, of course, is unavailable to Berkeley precisely because he rejects the suggestion that there is any such external reality. In fact, given that Berkeley simply identifies physical objects with ideas in the mind, and given that I am
currently having particularly vivid ideas of pink elephants, it would seem to follow that my pink elephants are real physical objects.

Clearly, this won’t do. How, then, does Berkeley distinguish between illusion and reality? How can he allow that my pink elephants are not real physical objects?

He says that not all ideas are of things that are real. Our ideas of real things, suggests Berkeley, are far more vivid than those ideas we conjure up with our imaginations. Berkeley also maintains that our ideas of real things are also ideas over which we have no voluntary control being put into our minds by God. The imagination, by contrast, is free to conjure up whatever it likes.

These suggestions do not go quite far enough in accounting for all perceptual errors, however. After all, nightmares can be very vivid indeed – so vivid we mistake them for reality. And they are terrifying precisely because they are beyond our

So how else might the real and the merely illusory differ? Berkeley adds that our ideas of real things have a constancy and regularity to them – indeed, they appear to be governed by laws (such as the laws of gravity). Illusions and hallucinations, on
the other hand, fail to fit in with our other experiences in a coherent way. When I hallucinate pink elephants cavorting around my lampshade, these experiences stand out like a sore thumb so far as the texture of the rest of my experience is concerned.
Here, suggests Berkeley, lies a further difference between those things that are real and those that are merely illusory.

Here, too, Berkeley’s explanation of the difference between illusion and reality seems inadequate. Surely someone might have a vivid but unremarkable dream that fits into the rest of their experience in just the way Berkeley describes. They might dream that they got up in the night for a glass of water, for example, when in reality they remained in bed. Berkeley has a hard time accounting for the possibility of this sort of

Few philosophers nowadays are idealists. Still, while almost every contemporary philosopher rejects Berkeley’s conclusions, they acknowledge that many of the points Berkeley makes in attempting to justify those conclusions are both insightful and thought provoking.


Paul P. Mealing said…
I must buy that particular book one day, seriously. I keep putting it off for books on science and mathematics, my other passions.

I jumped to your last paragraph in my head before I got there. When you were talking about imagining non-existent entities, I immediately thought of dreams and also images we conjure up whilst reading novels.

An extreme version of the Copenhagen version of quantum mechanics effectively gives the same version of ‘reality’ as Berkeley’s. John Wheeler speculated that the entire universe could be a giant cosmic quantum loop based on Bohr’s interpretation, which Paul Davies explores in The Goldilocks Enigma (which I know you’ve read).

What I find interesting is that it’s obvious that the universe has existed without consciousness and will continue to do so after consciousness is extinct. But, without consciousness, it’s a non-event. And without consciousness there is no reason for God to exist. If God exists then it only exists because we exist.

Regards, Paul.
Unknown said…
Berkeley had obviously never been married. Let us postulate for a moment that I have recently returned from a business trip; unpacked my suitcase, put away my things, and having left the suitcase in the path of the doorway, turning aside to prepare my shower. Doing so, I promptly forget the suitcase, at which point it ceases to exist. If all reality is perception, my reality will soon be subject to a shocking crack on the noggin. A blow delivered by my wife who, her arms full of folded laundry, has tripped ovver my non-existent suitcase which yet persists despite the fact it does not; because I have ceased percieving it and my wife could not percieve it because her arms were full of laundry, yet it has insisted on existing in a way that has independantly affected the reality of two human beings apparently unaware of its existence.
Unknown said…
Berkeley was never married or he would have known his logic was faulty. Let us postulate for a moment, that I have recently returned from a business trip. Upon returning home, I proceed to my bedroom to unpack, put away my things and, turning to the bathroom and the shower within, promptly lose sight of my suitcase which I have left sitting in the path of the bedroom doorway, at which moment, it ceases to exist. This stateless non-existence does not prevent my wife from cracking me, at her first opportunity, on the noggin however, when she, arms full of laundry and incapable of percieving the suitcase, trips over it and takes a tumble. Now, my fomerly non-existent wife walks soundlessly up from behind me and indeed does deliver a very real and painful reminder that my suitcase in fact does exist outside my perception.
The likely argument will follow that Berkeley's assertion that 'God's' continuous perception of all things has caused the suitcase to persist... does this then make God responsible for the suitcase tripping up my wife and not I? Am I to blame for wrongly and safely assuming that because I do not percieve the suitcase it does not pose a threat of tripping to my non-existent wife? If I were a dutiful husband, thoughtful and caring of my non-existent wife's unnecessary safety, would not I have gone to the extra effort to put the suitcase away and remove it as a clear and present danger to the unwary non-existent passerby?
The real discussion should be about Berkeley's translation of faith-based perception couched in the prose of the linguistic logic of emerging fact based perception, as a repudiation of Scientific Logic. A dialectic, in essence; fighting fire with fire. A fruitless enterprise on his part, but a superb precursor to Shroedinger's Cat. For Berkeley to retreat from his self-evidently empty logic by the sorry expedient of asserting that because God sees all, all that is unreal is actually real because God percieves it. It is nothing less than a surrender to the God of Scientific Logic: I am, therefore I exist. I am that I am. 1 = 1.
It is none-the-less a very dangerous path down which to lead humanity. To suggest that all that I do not perceive is not real will inevitably devolve into all that I may do/have done/no longer percieve does not and never has existed and no longer is because it is not real now. Inviting every kind of evil, perfidy, wickedness, and licentiousness absent any an all responsibility. Berkeley should have been more worried about the church pronouncing him anathema; deducing from his postulate that God, because he cannot be perceived is therefore not real.

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