What makes a classic?

Last week, I criticized Mur Lafferty for attempting to dismiss some of the classics of the genre unread. Reading some of the comments on that post got me to thinking about an obvious question: what makes a classic of the genre? Obviously, an ability to stand the test of time is the most important element in defining a classic, as a brief perusal of the bestsellers of 100 years ago, or even 50 years ago will demonstrate, but there must be more to it than simple longevity since there are no shortage of unread classics, both within and without the SF/F genre. Is there some sort of magic formula that allows us to distinguish between the merely popular and the temporally transcendent? We know that sales quantities are both objective and incapable of determining literary greatness, but does this mean that greatness is entirely subjective or are there some reasonably objective elements involved?

Read the rest at The Black Gate.