Art is indefinable because of its constant evolution. This evolution has taken place because art inherently has an aspect of human contribution and, therefore, is subject to progression in ways as numerous as its very creator's changes. There are two main reasons that illustrate the fact that art is indefinable. These reasons are that art has a changing role in society, and that the various production methods of art endlessly transform.
The first of the reasons is crucial because it shows that art, as a result of the human artist, will necessarily change when its human maker changes. This is what allows for art to have a specific time setting in which it will have a particular meaning and importance. Thus, because of the infusion of an artist into their art, the piece will in some degree convey a glimpse of a changing social environment. This environment may never be relevant again after that specific point in time, indirectly illustrating history and change.
The second reason for illustration again focuses on how the involvement of procedural evolution does not allow for the permanent defining of art, in this case in regards to the applied art production methods. Most recently computers have revolutionized some forms of art. It has also created a new movement through the application of technology in new and unique ways to the creation process. If art had been permanently defined prior to this new use of computers it would have stifled what has become a new and exciting branch of art, and as a consequence could have limited future productions from unimagined advancements.
An inflexible and permanent definition of art as a result of the aforementioned reasons is detrimental to the progression of art that is crucial to its very continuance. And as a result of this constant evolution art is indefinable.