Reality.

Sully points out conservative David Frum's about-face on same-sex marriage:

Most conservatives have reacted with calm -- if not outright approval -- to New York's dramatic decision. Why? The short answer is that the case against same-sex marriage has been tested against reality. The case has not passed its test.

That's all sweet and cuddly but it'd be nice if "conservatives" worried about reality elsewhere.  For instance, how many more decades of watching trickle-down economics fail miserably have to go by before they decide it fails the "reality test"?