Louise Anthony's reply is absolutely right, though the problem of other minds will be always with us no doubt. I wonder whether there is something else in addition in your mind that lies behind the question. Are you suggesting that whenever I am conscious there is a very interesting cause in the external world - the consciousness of others? So, for example, when I catch someone's eye, or when I become aware of the intelligence embodied in the design of a building, I become conscious. I think that there is truth to this interesting empirical proposal, but I wonder whether what is happening is that I become more conscious than I was in these cases, or conscious in a new way. A certain amount of education involves this, and, as Nagel pointed out a long time ago, the consciousness of mutual desire does too. But presumably there has to be a basis of consciousness already, or I could not become aware of anything conscious tugging at me from the external world. There has to be a consciousness there to be...

"Opposite" is not in this connection a very well-defined word. "Complementary" is more precise, but then we should inquire: physical additive complementary, i.e. such as to cancel the test colour in light superposition and produce neutral or white; physical subtractive complementary, i.e. such as to cancel the test colour in pigment mixing and produce neutral or black; psychologically complementary - it is unclear what this would mean, but it could have to do with the placing of the test in a colour space based on the psychologycal "unitary" hues, i.e. those that do not look as though they contain a "trace" of any other hue in the space. There are some interesting studies of brown, and one of them (I think) is my own, in Jonathan Westphal, Colour: A Philosophical Introduction, Blackwell, Oxford, 1991 - the chapter on "Brown". Are we allowed to sound our own trumpets on this website? I'm not sure, but anyway this might get you started. The thing to remember is that brown surfaces have roughly the same...

In physics there is the Zeroth Law of Thermodynamics, a transitivity of temperatures. If A and B have the same temperature, and B and C have the same temperature, then A and C have the same temperature. (A could be melting ice, B expanding mercury, and C bourbon at the correct temperature.) The name for the Law was devised after the names for the three laws of thermodynamics had become standard.

"Let's agree that something is art if the art world views it as art." I think that we shouldn't agree to this proposition, precisely because it does seem that it has the relativistic implications you describe. So I take your question to be, "Does the institutional theory of art, e.g. George Dickie's (the theory that something becomes art if and only if the 'artworld' gives it this status) have the implication that something can be a work of art at one time and then not be at another?" The answer the institutional theory gives is that it is the totality of all the little artworlds at particular times and places inclusion in which at any time makes something art. Does this answer work? Does it make indeterminate today whether this is a work of art, as whether it is seems to depend on the possible response of some future art scene. Some ugly Greek pot is displayed in a museum by lovers of Culture today, so it was art even if the people who saw it at the time it was made dismissed it as crassly...

This is a fairly frequent concern. The correct answer is that there is a sense of "colours" in which black and white are not colours (they are not chromatic colours) and a sense in which they are colours (they are achromatic colours). So if we count the achromatic colours (black, white and grey) as colours, then black and white are colours. (Brown is an interesting case, as it is a colour which is partially achromatic.) In the same way, we can ask whether zero and infinity are numbers. Usually they are treated as numbers, and they have their own mathematical symbols. We can manipulate them in calculations and so forth. But in another sense "zero" denotes the absence of a number, and so does the symbol for an infinite number. Q: "How many chickens were there in the kitchen?" A: "A number." Q: "What is the number?" A: "Zero"! Aristotle's view was that the smallest number is two, as one of something is not a number of somethings. "There were a number of people there." How many?" "One." In...

There are lots of things that don't exist that we can conceive of, most obviously fictional characters and objects, though our conceptions of them may be less detailed or thorough-going than our conceptions of things that do exist. (They may also be more detailed. We may know more about the characters in Tolstoy's novels than about some real people.) We can conceive of an elixir of youth, though there may not be such a thing, for example. And if consciousness exists, where does it go when you are meditating or fast asleep? Well, why does it have to go anywhere? Isn't it more like a noise, say, that is real enough in its way, but which just shuts down or disappears when the thing making it go stops, and doesn't have to go anywhere? Of course if the individual consciousness is a sort of stuff, rather than a kind of attention, a substance even, in the philosophical sense, then presumably it keeps on going even if it isn't with you any more. But it is difficult to see how that would work; what...

It is reasonably unclear what "mental" and "mind" mean overall, though it is clear that whatever it is the intellect is a central part of it, and so if some philosophers want to insist that the mind is to be taken as the conscious mind, there is nothing to stop them, and there is a reasonable point to their stipulation. But even as early as the turn of the 17c, Leibniz accepted unconscious thinking, and there is a very strong case to answer here. Not answering it can make the stipulation look very arbitrary. Locke died in 1704, by the way, and Berkeley in 1753, so they did not hold the view that you attribute to them "in the later 1700s". And Hume's philosophical writing was done early on in his career; the Treatise and the Inquiry were published before 1750.

If I dream that I am in Lisbon, it does not follow that I am, and I may not be. Nor does it follow that I am not, of course. But if I say that I dreamed that I was in Lisbon last night, this may be one way of saying that I was not in Lisbon. If then I say that I dreamed (in a "ground-floor" dream) that I dreamed something (in a second-level dream, so to speak), it seems to follow that I did not; I did not produce a dream within my dream. And it seems to me hard to see how I could. For the contrast is between dreams and reality, and there is no reality for the second-level dreams to return to. There are just sequences of images and dreamed descriptions of them. If I dream that I have a dream, the second thing I have is not a dream within a dream, but just part of the ground-floor dream.

Depression used to be classified in two forms: endogenous ("originating from within") and reactive. There was an obvious point to this way of classifying things, but a different way has been suggested recently. The newer way is to distinguish between cases in which depression, the medical condition, is present, with its bodily neurophysiological causes, and cases in which only symptoms of depression are present, and which lack the underlying neurophysiological cause. The symptoms might however be produced by some environmental cause, a depressing incident, a personal loss or a major setback in life, without the marked and persisting change in the biochemistry of the brain which is believed to underly the medical condition. The significant difference between the two kinds of condition seems to be that changes in the environment typically don’t touch the medical condition. That may even be a defining characteristic of the condition, as well the usual things doctors look for, such as failure of concentration...

A mathematician might find his feelings engaged by certain questions. Sir Andrew Wiles was passionate about Fermat's Last Theorem from the age of about ten, I believe. (Say, by contrast, that he took little interest in statistics. Perhaps statistics even disgusts him.) Does any of this "corrupt the purity of the logic" of his (rather long) answer to the question how to prove Fermat's Theorem? No, it just powered his interest in mathematics. Besides, why isn't it possible to be inspired and motivated by a thought or an ideal? The ten-year old Wiles had the thought, 'I will prove the Theorem', and this motivated him and engaged his feelings - and the grown-up Wiles did prove the Theorem. The purity of his logic was perhaps even assisted by his passion.