One of the most fundamental distinctions in legal theory is that between "positive legal theory" and "normative legal theory." This post provides a very brief introduction to the distinction, aimed at law students (especially first years) with an interest in legal theory.
The core idea of the distinction between positive and normative legal theory is simple: positive legal theory seeks to explain what the law is and why it is that way, and how laws affect the world, whereas normative legal theories tell us what the law ought to be. Thus, a positive theory of tort law might seek to explain what causal forces have produced the existing principles of tort law, whereas a normative theory of tort law would tell us what rules of tort liability would be best, right, or justifiable. Or more simply: positive legal theories are about facts and normative legal theories are about values.
Positive Legal Theory Sometimes, the notion of positive legal theory is presented in an oversimplified way--as if there were a single, well-defined type of theory that counted as positive. In fact, the phrase "positive legal theory" is used in a variety of ways. The one thing that positive legal theories have in common is that they are not normative. Nonetheless, there are three characteristic type of positive legal theory that can be identified:
Positive Legal Theory Type 1: Doctrinal Theories--The first kind of legal theory that is called "positive" is quite simply a theory of what the content of a particular field of legal doctrine is. Thus, a theory of the freedom of speech might simply seek to explain the shape of existing first amendment doctrine. Or a theory of hearsay rule might seek to provide an account of the rule and exceptions that explains and accurately predicts particular applications of the rule. Doctrinal legal theories are responsive to questions like, "What are the principles that shape this area of the law?" or "Can these cases be explained by some underlying theory?"
Positive Legal Theory Type 2: Explanatory Theories--The second kind of legal theory to which the label "positive" is applied are explanatory theories--theories about why the law is the way it is. For example, a very simple Marxist theory might state that the content of the law can best be explained by the interests of the ruling class. Some legal economists have tried to argue that common-law rules are efficient, because there is "evolutionary pressure" on inefficient legal rules.
Positive Legal Theory Type 3: Effects Theories--The third kind of legal theories that are referred to as "positive" are theories about the consequences that will be produced by a given regime of legal rules. This is the sense of "positive theory" that is most frequently invoked by legal economists. The question --"What effects will a strict liability regime (as opposed to a negligence) regime have on the manufacturers of consumer products?"--can be answered by a legal theory that is positive in the sense that it predicts behavior but does not explicitly evaluate the desirability of the rule.
Ideal versus Nonideal Theory
Some normative legal theories are "ideal"--that is, they are theories about what the best legal rule would be in the world in which everything was politically possible, the law could be adequately enforced, and other legal rules that interact with the subject of the theory could be adjusted to produce the best overall system. Other normative legal theories are "nonideal"--that is, they are theories that assume a variety of constraints on the choice of legal rules. For example, a nonideal theory might take into account political feasibility or it might take into account the possibility that the system would not provide an optimal level of enforcement for the rule that would otherwise be best. The Legal Theory Lexicon entry on second best explores these ideas in greater detail.
Justificatory Theories and Critical Theories Normative legal theories also vary in their "attitude" towards the status quo. You are likely to encounter normative legal theories that start with the question, "What is the best justification that be given for such and such a legal rule?" These justificatory theories have a limited purpose. They do not address the ultimate question, "What is the best legal rule?" On the other hand, many legal theories have the opposite purpose--the critique of existing legal doctrine. Thus, a critical theory might enumerate all of the criticisms that could be made of an existing legal rule--even though some of the criticisms may rest on inconsistent premises.
Positive Theory in the Service of Normative Theory
One relationship is clear and straightforward. Many normative theories underdetermine what the legal rules should be in the absence of substantial information about the effects of the rules. This is most obvious in the case of utilitarian theories, where information about consequences does all the real work of determining which legal rule is best. For normative theories like utilitarianism, positive theory performs an essential service. Without a positive account of the effects of a given rule choice, utilitarianism has nothing to say about what rule is best.
Positive Theory as a Constraint on Normative Theory Another relatively noncontroversial relationship between positive and normative legal theories arises when a positive theory that explains why the law has the shape that it does, is taken as imposing a constraint on normative theory. For example, public choice theory makes certain predictions about how legislatures will act in response to various incentives. Some legal rules that might be justified by ideal normative legal theory may be considered "unrealistic" in light of positive theory. In cases like this, positive legal theory provides constraints that limit the options available to normative theory.
Interpretivism and "Law as Integrity" There is another, more controversial, way that positive and normative legal theory can interact. Ronald Dworkin's theory of law, "law as integrity," attempts to combine the aims of positive doctrinal theory and normative theory. The idea is that a legal theory should both fit and justify the existing legal landscape. Thus, a Dworkinian theory of the freedom of speech would need to both fit the contours of the Supreme Court's decisions and justify those decisions. Of those interpretations of free speech doctrine that fit the legal topography, Dworkin maintains that judges should select that interpretation that makes the existing law, "the best that it can be." Dworkin's view of legal theory blurs the line between positive and normative legal theory--essentially combining the enterprises that I have called positive doctrinal theory and justificatory normative theory. As you might imagine, this is hugely controversial--although that is a topic for another post.