In ‘What Has Philosophy to Learn from Tort Law?’, Bernard Williams reaffirms J. L. Austin’s suggestion that philosophy might learn from tort law ‘the difference between practical reality and philosophical frivolity’. Yet while Austin regarded tort law as just another repository of time-tested concepts, on a par with common sense as represented by a dictionary, Williams argues that ‘the use of certain ideas in the law does more to show that those ideas have strength than is done by the mere (...) fact that they are part of the currency of common sense’. But what does it mean to show that ideas or concepts ‘have strength’? How does conceptual strength relate to the distinction between practical reality and philosophical frivolity? And what special features of the law are supposed to make it a better test of conceptual strength than common sense? In this chapter, I reconstruct and develop Williams’s answers to these questions. I show why Williams believes that we need to test the concepts forming the currency of common sense against practical reality as embodied by legal practice; I identify seven features of tort law that make it particularly suitable to act as such a test; I distinguish three respects in which concepts can show strength, and unpack Williams’s metaphor of microwave-resistant concepts: concepts capable of holding and presenting material for intense critical scrutiny without succumbing to it themselves; lastly, I show how philosophy can learn to identify systematically weak concepts, and the limits of otherwise valuable concepts, by considering which concepts fail the test. (shrink)