That, at least, is the advice we all encounter ear­ly on when first we try our hand at invest­ing. Home­spun though it may sound, the idea has aca­d­e­m­ic roots: the Effi­cient Mar­ket Hypoth­e­sis, as the econ­o­mists call it, holds that the prices in any finan­cial mar­ket already reflect all avail­able infor­ma­tion rel­e­vant to what’s being trad­ed with­in them. In the case of the stock mar­ket, for exam­ple, every­thing known — or indeed, know­able — about the future prospects of a par­tic­u­lar com­pa­ny is already incor­po­rat­ed into its stock price, or might as well be. All this may not sound like the usu­al ter­rain of Errol Mor­ris, whose pre­vi­ous doc­u­men­taries have pro­filed every­one from pet ceme­tery oper­a­tors to for­mer U.S. sec­re­taries of defense to Stephen Hawk­ing. After all, most any field has some con­nec­tion to the inex­haustible sub­ject of how we know, what we know, and what we can’t know.