The second fundamental result analyzes the consequences of rationality and common belief/knowledge of rationality in extensive games (i.e., trees instead of matrices). Here, the most well-known solution concept is the so-called subgame perfect equilibrium, also known as backward induction in games of perfect information. The epistemic characterization of this solution concept is in terms of “substantive rationality” and common belief that all players are substantively rational (cf. also Vanderschraaf &amp; Sillari 2009: sec. 2.8). The main point that we highlight in this section, which is by now widely acknowledged in the literature, is: