Scepticism asked whether you are entitled to believe in an external world. This lesson asks a different question that is constantly confused with it: what the world is, assuming there is one. A sceptic says you cannot know the table is there. An idealist says the table is there and is made of something other than what you assumed. These are not the same position and they have almost nothing to do with each other.
Three positions
Realism about the external world holds that physical objects exist and have their properties independently of any mind. The table would be there, with its shape and mass, if every observer vanished. This is what almost everyone believes before being asked and what 79.5 percent of philosophers endorsed in the survey cited in lesson one.
Idealism holds that what exists is minds and their contents, and that physical objects are constituted by experience rather than causing it. The table is real, is public, and will hurt if you walk into it, and it is not made of mind-independent stuff.
Phenomenalism is the more cautious descendant, associated with John Stuart Mill, who defined matter as the permanent possibility of sensation. Statements about tables are analysed as statements about what would be experienced under various conditions. The table in the empty room is not a thing sitting there unobserved, it is the truth that anyone who walked in would see one.
The difference from scepticism is worth restating because it is where most readers go wrong. All three positions agree that you can know perfectly well there is a table. They disagree about what that knowledge is knowledge of.
Why the question is not silly
The usual first reaction is that idealism is obviously false and that anyone can refute it by kicking something, which is what Samuel Johnson did in 1763 when Boswell reported Berkeley's view: he kicked a large stone and said "I refute it thus".
Johnson's kick refutes nothing, and seeing why is the beginning of taking the question seriously. Berkeley's claim was never that stones are insubstantial or that you can walk through them. He held that stones are exactly as hard, as public and as kickable as everyone thinks, and that this hardness consists in a regular structure of experience rather than in mind-independent matter. The kick produces precisely the sensation Berkeley predicts. It discriminates between the hypotheses not at all, which by lesson five means it is not evidence.
The serious route in is the one Locke opened and Berkeley closed. Locke, in the Essay Concerning Human Understanding (1690), distinguished primary qualities, which objects have in themselves, from secondary qualities, which are powers to produce sensations in us. Shape, size, motion and solidity are primary. Colour, sound, taste and warmth are secondary: the redness is not in the apple, it is in you, and what is in the apple is a surface that reflects certain wavelengths.
Galileo had said much the same in Il Saggiatore (1623), and the distinction survives in modern physics, which describes the world in terms that never mention how anything looks.
Example. A friend says the primary and secondary quality distinction shows that science has already proved idealism, since it turns out colour is in the mind. What has gone wrong?
The distinction does the opposite. It removes colour from the object and leaves shape, mass and motion firmly in it, which is a realist conclusion: the world is not as it appears, and it is still there, having properties of its own. The friend has taken the demotion of one class of properties as though it were the demotion of all of them. Notice that this is exactly the step Berkeley did take, deliberately and with an argument, which is what makes him interesting: he agreed with Locke about colour and then asked why shape should be treated differently. The friend is helping themselves to Berkeley's conclusion without Berkeley's reasoning.
Now you. Why does the fact that physics describes the world without mentioning appearances not settle the question in the realist's favour?
Answer
Because a description can be perfectly accurate about structure while remaining silent about what has the structure. Physics tells you how mass, charge and spacetime curvature relate to one another, and every one of those terms is defined by its relations to the others and ultimately to measurements. Bertrand Russell pressed this point in The Analysis of Matter (1927): physics gives us the structure of the world and not its intrinsic nature, which is why panpsychism in lesson eight is a live option rather than a joke. So the absence of appearance-talk in physics shows that the world is not built from appearances, and leaves entirely open what it is built from. That gap is the space every non-realist position occupies.
Berkeley's argument
George Berkeley published A Treatise Concerning the Principles of Human Knowledge in 1710. His slogan is esse est percipi, to be is to be perceived, and his case has two parts.
The first attacks Locke directly. If secondary qualities are mind-dependent because they vary with the observer, then so are primary ones. Put one hand in hot water and one in cold, then both in tepid: the same water feels warm and cool, which is the standard argument about temperature. But size varies with distance, shape varies with angle, and motion varies with the observer's own motion. Whatever argument demotes colour demotes shape. Locke has drawn a line he cannot defend.
The second is the master argument, and it is the one worth sitting with. Berkeley challenges you to conceive of a tree existing unperceived. You try, and you form a mental image of a tree in an empty forest. But in doing so you are conceiving of it, so there is a mind in the picture after all, namely yours. Berkeley concludes that the very attempt to think of something unperceived defeats itself.
Nearly everyone senses that something is wrong here, and saying what takes care. The standard diagnosis is that Berkeley conflates the object of thought with the act of thinking. When I imagine an unperceived tree, the tree I imagine is unperceived even though my imagining is a perception. Compare: I can think about a thought I am not having, and there is no contradiction, because the thought I am thinking about is not the thought I am thinking with. The argument trades on a use and mention confusion.
That diagnosis is now standard and it does not rescue realism by itself, since it defeats one argument for idealism rather than establishing that matter exists. Berkeley's first argument, against Locke's line, remains and is harder.
Example. Take Berkeley's first argument seriously. Water at one temperature feels warm to a cold hand and cool to a hot one, so warmth is in the observer. A coin looks round from above and elliptical from an angle, so shape is in the observer too. What can the realist say?
That the two cases are not parallel, and the difference is that the variation in shape is systematic and predictable from the geometry of the viewing position, while the variation in warmth is not predictable from anything about the water. Knowing the coin is round plus the angle, you can compute the ellipse; knowing the water is at 30 degrees, you cannot compute whether it feels warm without also knowing the hand. So shape supports a single objective assignment that explains every appearance, and temperature does not. That is a real reply and it concedes Berkeley something: the line between primary and secondary is not where Locke drew it by listing qualities, it is drawn by which properties admit of an observer-independent assignment that predicts the observations.
Now you. Does the reply above survive relativity, where length and duration genuinely depend on the observer's motion?
Answer
Yes, and the case sharpens it. In relativity the length of a rod really does differ between reference frames, so there is no single objective length in the way Locke assumed. What there is instead is the spacetime interval, which every observer agrees on, plus a transformation rule that predicts each frame's measurement exactly. The realist reply does not require that the naive property be observer-independent, only that there be some observer-independent structure from which every appearance follows. Relativity supplies exactly that, and it is a better example of the reply than the coin is, because it shows the strategy working in a case where the intuitive property genuinely fails. The moral is that physics keeps replacing apparent properties with invariant ones rather than dissolving them into minds.
Where this leaves us
Berkeley had an answer to the obvious objection, which is what happens to the furniture when nobody is in the room. His answer was God, who perceives everything continuously, which is why the world is stable and public rather than flickering in and out as people enter and leave. That is a serious move within his system and it makes idealism dependent on the arguments of lessons ten and eleven, which is a heavy dependency.
Phenomenalism tried to keep the idealism without the deity, and ran into a technical problem it never solved: analysing "there is a table in the empty room" into claims about what would be experienced requires specifying the conditions of observation, and those specifications keep having to mention physical objects, such as an observer with working eyes standing in a lit room. The analysis presupposes what it was supposed to eliminate.
So the realist is in decent shape, not because idealism is absurd but because its best versions either need God or cannot be stated without circularity. That is a conclusion reached by weighing rather than by kicking, which is the whole point.
Example. Someone says: "If the simulation hypothesis were true, idealism would be true, since everything would be information in a computer rather than matter." Is that right?
No, and the confusion is instructive. A simulation is run on a computer, which is a physical object in some world, so the simulated table is constituted by physical states of that machine rather than by anyone's experience. That is a claim about what matter is made of at bottom, not a claim that matter is mental, and it is closest to a realism with an unexpected inventory. It would become idealism only if the computer itself were held to be mental, which requires a separate argument. Notice that this repeats the distinction the lesson opened with: changing the story about the substrate is not the same as denying there is a substrate.
Now you. Which of the three positions is most threatened if consciousness turns out to be entirely explicable in physical terms?
Answer
Idealism, by a large margin. Realism is untouched and arguably strengthened, since a physical account of the one phenomenon that looked non-physical removes the last apparent exception. Phenomenalism is damaged but survives as a thesis about meaning rather than about what exists. Idealism is in real trouble, because it holds that minds are fundamental and everything else is constituted out of them, and an explanation of mind in physical terms inverts that dependency: if experiences are what certain physical systems do, they cannot be what physical systems are made of. That is why the next lesson is not a digression. The status of consciousness is the pivot on which idealism, the simulation hypothesis and one popular argument for God all turn.