I like the general definition offered above and feel that it creates about as much clarity as we might achieve on the issue:
Traditionally, though, art should have properties of representation, expression, and formality.
However, after registering agreement with this definition and ceding to it the best likelihood of being a working definition, we can also plainly see that the terms used in the definition are not exactly specific. Representation is a rather loose term when we consider contemporary art.
Mark Rothko might have said he was representing emotional planes with his color fields. Jackson Pollock may have been representing the same thing with his drip style. It seems clear to me that these artists were creating works of art, yet the representational element becomes abstract, potentially, to the breaking point when we try to apply it to work that is actually known as "non-representational" art.
Art should address aesthetics and appeal in some way to the senses. Not everyone appreciates the same things in the same way. Some might disagree over appeal and/or value, but the aesthetics are what make this an issue of judgement. Much like literature the fact that literature can make many people respond in many different ways makes it literature.
I have to agree with #4 - art is impossible to set a boundary around or to delineate some sort of essential definition, as art really is any object or music that can provoke strong reactions in its audience. However, this is in itself to vague and broad for a useful definition. Personally I think what people consider to be "art" says more about them than it does about what they are referring to.
In my opinion, there is no way to define art.
This is because there are so many diverse types of expression that some people would say are "art." There is no way to create a definition that includes Beethoven's 5th Symphony while also including the urinal mentioned in Post #2. Both of these, and everything in between, are meant to evoke emotions or thought. But this cannot be the definition of art. If it were, every spoken or written word would be art.
I would suggest that you read the following page to get a really good discussion of this question:
The philosophical definition of art is a subject of controversy. Traditionally, though, art should have properties of representation, expression, and formality. What this means is that in order to be considered art, the artwork must represent something else, must be an expression of an artist, and must be presented in a formal context.
This definition prevents people from claiming that natural beings/items are art. For instance, although a leaf on a tree may be beautiful, it cannot in itself be art, though a painting of a leaf can be art. Items produced for a particular purpose/use can also not be considered art under this traditional definition.
The reason for controversy about the traditional definitions of art is that artists have expanded the definition over time. Looking at modern art, many pieces may not fit the traditional definition. This article shows one artist who stretched the bounds of what is considered art. He displayed a urinal as an art piece, calling it "found art". http://en.wikipedia.org/wiki/Fountain_(Duchamp)
After having long acquired great prestige as a dignified activity, and after having occurred in diverse sacred and profane contexts, it is still in need of justification. It is not that art has lost its power to move us, but rather, that judgment regarding art is so uncertain that any assertion about it is incapable of achieving a general agreement.
In the same way, our own opinions concerning artworks have become uncertain even to ourselves. This is true not only for art lovers, but also for scholars, including those in the domains of music, dance, and theater.
Philosophical hermeneutics is a branch of philosophy that attempts to legitimate artistic expression on the basis of a philosophical consideration which, going back to ancient times and recovering medieval and modern philosophical thought, has brought into light new ways to understand cultural phenomena. Philosophical hermeneutics has been able to rescue art from the oblivion to which modern society has confined it under a supposed pure aesthetic judgment, separating it from the vital contexts from which it was originally associated and from which it acquired its full sense.