Can't we just say that there's no such thing as truth? Our epistemology, at least for model-dependent realists, assumes that there's no such baseline -- there's no static reality against which we must measure the success of our perceptions. Reality is instead just valued against a floating relationship between the knowledge and the observation's moment in space-time, a function of the relationship itself as well as the necessity and utility that motivated the acquisition of that particular knowledge. Empiricism could indeed answer that sort of question, anyway. Instead of what is our foundation for truth, it only has to tell us what is the best way to arrive at knowledge that ends up being most reliable to suit the motive for the observation.