What qualifies a classic?

The "classic" definition has puzzled me. Most readers and students seem to agree on an unspoken definition for term. We use the word to connote high literary merit, longevity, and model form. Our classics are our archetypes. Educated readers, we understand, study and reference the classics. Half-guiltily I admit that I purchase leather-bound classics from antique shops for the sole purpose of collecting them on my bookshelves, the marks of a true scholar. Reading them would be ideal, of course, but at least I have them on hand to imply my reading.

Why do I succumb to these standards? Should I succumb to these standards? What is valuable reading material?

Charles Augustin Saint Beuve writes that "a true classic, as I should like to hear it defined, is an author who has enriched the human mind, increased its treasure, and caused it to advance a step" (http://www.bartleby.com/32/202.html). I agree with his definition. Maybe a classic isn't a 'model in any language whatsoever,' and maybe historical longevity isn't its primary characteristic, but in its own way it enhances and stretches the thinking of a pocket of humanity.

What I find interesting is that Charles Augustin defines a class as a "who," not a "what." He writes that a classic is one "who has expressed his thought, observation, or invention, in no matter what form." So, then it seems that the author's personal quality (of writing?) is what characterizes 'classic.' The writer is the classic, not necessarily the work he produces. So does that mean that Milton, for example, always produces 'classic' work by virtue of being a classic himself? Can different works from the same author be classified as both 'classic' and 'non-classic'?

Jane Austen comes to mind. A recent, 2007 movie version of Austen’s lesser-known novel Mansfield Park exists, and I’ve prolonged renting it until I read the book. I haven’t read much of Austen after severely disliking Sense and Sensibility, so this will mark yet another attempt to appreciate her work beyond Pride and Prejudice. If Austen herself is classic, then it doesn't seem to follow that all of her work be classic. Pride and Prejudice certainly seems more classic than Mansfield Park. So here's my question: should the classic label refer to the author or to the work?