Yale law professor Ian Ayres explains one of the basic analytical tools lawyers use to evaluate rules and decisions: the difference between judging an action ex ante, before the outcome is known, and ex post, after the fact. Drawing on examples from contract and tort reasoning, Ayres shows how a decision that looks reasonable looking forward can look negligent or unfair in hindsight, and why courts and legal scholars have to be explicit about which vantage point they are using. The lecture is part of Yale's Coursera course A Law Student's Toolkit, aimed at giving prospective and current law students the vocabulary and reasoning habits used throughout legal argument. It runs short and stays focused, building the ex ante and ex post distinction step by step with plain examples rather than case citations, so it works as a quick primer on a concept that recurs across contracts, torts, and regulatory policy.