Rorty said that "truth" is "a compliment paid to sentences that seem to be paying their way and that fit with other sentences which are doing so".
It's essentially what Krugman praises in Keynesian theory. It seems to work. As Krugman writes, "We have a model of the way the world works, and the world does indeed seem to work that way."
Any ultimate answer needs more than that, of course. But that attitude is the heart of any scientific claim that becomes broadly accepted: the world seems to work the way it says it will work.