Many complex systems--from immensely complicated ecosystems to minute assemblages of molecules--surprise us with their simple behavior. Consider, for instance, the snowflake, in which a great number of water molecules arrange themselves in patterns with six-way symmetry. How is it that molecules moving seemingly at random become organized according to the simple, six-fold rule? How do the comings, goings, meetings, and eatings of individual animals add up to the simple dynamics of ecosystem populations? More generally, how does complex and seemingly capricious microbehavior generate...

What does it mean for scientists to truly understand, rather than to merely describe, how the world works? Michael Strevens proposes a novel theory of scientific explanation and understanding that overhauls and augments the familiar causal approach to explanation. What is replaced is the test for explanatorily relevant causal information: Strevens discards the usual criterion of counterfactual dependence in favor of a criterion that turns on a process of progressive abstraction away from a fully detailed, physical causal story. The augmentations include the introduction of a new,...