Let's agree that something is art if the art world views it as art. Many famous painters were refused exhibition for years, only to have their rejected works considered masterpieces at a later date. Others were considered great artists, only to be virtually forgotten. Operas go in and out of fashion. Same with literary works. Does this mean that the same works can be art at some points in time but not art at other points in time?

"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...

Are black and white colors, or not?

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...

If there is no such thing as consciousness, how can I conceive of consciousness, or of what consciousness must be like? Conversely, if consciousness exists, when did I "get" it, and where does it go when I'm meditating?

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...

In the later 1700's, many famous philosophers (Locke, Berkeley, Hume) held the 'transparency thesis', the view that all important mental contents could only be conscious. Is this position still defensible?

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.

Is it logically possible to have a dream within a dream? Or is there, as it were, only one "level" of dreaming?

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.

As someone who is clinically depressed, I have often wondered: philosophically speaking, is trying to treat depression wrong? People are depressed for a reason, possibly because life's pretty damned depressing once you get down to it. It seems to me that in plenty of cases, depression is a logical reaction to this planet, a rather depressing thought in and of itself. Despite the wars and the plagues and the genocides and the poverty and the seemingly countless other reasons for one to be depressed, people treat depression like a disease when it seems more like a perfectly acceptable reaction to the human condition. Treating depression like this appears to me as a rather unsubtle way of trying to trick people into believing everything is going to be okay when reality seems to contradict this. Any thoughts?

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...

I believe it was Hume who made the point that reason cannot motivate us, only our feelings can. Supposing that's true, I have a far-flung conclusion that seems to follow from that: when the panelists on this site choose which questions to answer, they're motivated by some emotion, not by reason. But doesn't this corrupt the purity of the logic of the answer? Perhaps not necessarily so, but isn't it likely that of the 2,600+ questions a good number have been tainted? How is it not the case?

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.

Doesn't time travel involve space travel too? If I travel back in time one year, say, in order to be in the same 'place' as I started, I'd need to travel across countless millions of miles of space, since the planet has moved during the last year. Since such instant space travel contradicts Einstein, how come so many philosophers seem to think it's possible? Martin, Wales, UK

You make a very interesting point. If time travel takes a second, then since a later Earth - say Earth in a year - might be zillions of miles away (i.e. more than 186,000), I must travel faster than light, which is impossible. But how long does my time travel take? How do we know that it takes a second? After all, if on the new or later Earth it is a year later, presumably it took me a year to get "there", the same amount of time as it took the Earth itself, even if it felt instantaneous. So there is a difficulty about the meaning of "How long does my time travel take?" If we move in time, or times moves past us, there is a difficulty about the concept of the speed of the movement. Movement in space is distance divided by time, so movement in time, or the movement of time, it seems, is time divided by time; and it is hard (as D.C. Williams pointed out ages ago in "The Myth of Passage") to attach any sense to this idea. This is the interest of your point for me; how do we attach sense to the speed of...

It has been suggested that the practice of Bonsai is an expression of animal chauvinism and does great harm to a tree by 'stunting' it. But aren't trees not sentient beings, and therefore the excising of branches, shoots and roots such that the tree thrives albeit substantially smaller than its genetic potential, is no different to the continual loss of roots, shoots and branches that occur under natural conditions?

There is the fact that it is possible to treat something badly or to damage it, whether or not it is sentient. A pair of shoes can be badly or well looked after, and it is wrong not to look after a good pair of shoes properly. A living thing like a hedge can be properly maintained or attended to, but something more is involved. Sometimes, it seems to me, the aesthetics of the "art object" (horrible phrase) can reflect what it actually is. So it is natural to trim a box hedge in one way, and a mixed hedge in another way, perhaps a less formal and more undulating, the shapes reflecting the different kinds of growth - hawthorn, or privet or beech. (A hedge that is full of straggly and unwanted sycamore looks awful, and the holes soon begin to show.) Pugs and poodles may be perfectly happy qua sentient beings, though often there are specific health problems with specific breeds, but it is perfectly coherent to object to the whole process of breeding and thinking of living things as being objects for our use ...