Legal Theory Lexicon 001: Ex Ante/Ex Post If I had to select only one theoretical tool for a first-year law student to master, it would be the ex post/ex ante distinction. (Of course, this is cheating, because there is a lot packed into the distinction.) The terminology comes from law and economics, and here is the basic idea:

• The ex post perspective is backward looking. From the ex post point of view, we ask questions like: Who acted badly and who acted well? Whose rights were violated? Roughly speaking, we associated the ex post perspective with fairness and rights. The ex post perspective in legal theory is also loosely connected with deontological approaches to moral theory. In general jurisprudence, we might associate the ex post perspective with legal formalism.
  
• The ex ante perspective is forward looking. From the ex ante point of view, we ask questions like: What affect will this rule have on the future? Will decision of a case in this way produce good or bad consequences? Again, roughly speaking we associate the ex ante perspective with policy and welfare. The ex ante perspective in legal theory is loosely connected with consequentialist (or utilitarian or welfarist) approaches to moral theory. In general jurisprudence, we might associate the ex ante perspective with legal instrumentalism (or legal realism).

Of course, this very basic introduction to the distinction is oversimplified. For example, a fairness-based theory of torts might consider future consequences in assessing legal rules, and even utilitarian legal theories must use ex ante information when evaluating particular cases.
Why is the distinction between ex ante and ex post so important? Because it marks an important theoretical divide between consequentialist and deontological approaches to legal theory. Consequentialists, we might say, simply don't care about the question whether A has violated the rights of B, for their own sake. Rather, a consequentialist cares about the consequences of attaching liability to those who act like A did. Ex ante, is a strict liability rule or a negligence rule more efficient? Deontologists, on the other hand, care very much about who has acted rightly and wrongly. In tort law, for example, corrective justice theories of tort are associated with the ex post perspective. A should be liable to B, only if A has acted wrongly.
If you are a first-year law student, you might make a habit of asking yourself questions like the following:

Is the rule in the case I've just read, just or fair from an ex post perspective?
Will the rule produce good consequences (as compared to the alternatives) from an ex ante perspective?