Introduction
What policy choices are feasible and which
options are "pie in the sky," "utopian," or "politically impossible"?
What is "necessary" and what is merely "probable"? The Legal Theory Lexicon includes two entries that deal with aspects of these questons Legal Theory Lexicon 011: Second Best and Legal Theory Lexicon 062: Path Dependency.
In this post, we will investigate the concepts of "possibility" and
"necessity." These ideas are ubiquitous in theoretical discourse, but
they are rarely defined or explicated. One way to get clear about
possibility and necessity is via the power philosophical idea
of a possible world, made famous by Leibniz and deployed in
contemporary metaphysics by Saul Kripke and others.
As always, the Legal Theory Lexicon
is aimed at law students, especially first year law students with an
interest in legal theory. Although "possible worlds" talk may sound
complex, the core ideas and terminology can easily be mastered.
Modality
Before
proceeding any further, we need to introduce an important term, the
adjective "modal" and its noun form "modality." These words are
probably unfamiliar to anyone who wasn't a philosophy major, but you
may dimly recall that "could" and "must" are called "modal verbs" in
texts on English grammar. "Could" is a modal verb because it expresses
possibility: if something could happen, it is possible. "Must" is a
modal verb because it expresses necessity: if something must happen, it
is necessary. When I use the word "modality" in this post, I am simply
referring to the general concept that includes both necessity and
possibility in all their forms.
Possible Worlds Semantics
The
phrase "possible worlds semantics" sounds pretty intimidating, but it
is really based on a very intuitive and simple set of ideas. Possible
worlds talk translates talk about possibility and necessity into talk
about possible worlds.
Begin with the notion of a
possible world. This notion is similar to the notion of a "state of
affairs," which may be familiar from economics. The philosophical idea
of a "possible world," is understood as a complete history of a whole
universe. If some thing is possible, say event X, we say that X occurs
in some possible world. Complimentary to the concept of possibility is
the concept of necessity. Let us say that an event Y is necessary if Y
occurs in all possible worlds; a proposition, p, is a necessary truth
if it is true in all possible worlds.
The next step is to add
the notion of the "actual world," where actual is an indexical term
that separates this world from all possible worlds. You may not be
familiar with the idea of indexical terms. "Here" is an indexical: its
meaning is indexed to the place where the word is uttered. Likewise, "now" is an indexical: Its meaning is indexed to the time the word "now" uttered. And when "actual" is used in the philosophical sense, its meaning is indexed to this possible world: the actual world is the possible world that we inhabit.
Varieties of Possibility
Now
that we have the idea of a possible world in place, we can talk about
different kinds of possibility. For example, "logical possibility"
requires only logical consistency, and, in the logical sense,
possibility is rarely implicated by debates in normative legal theory.
That is because almost anything that legal theorists would care to talk
about is logically possible: it is logically possible that the
Constitution will be amended next year to abolish the Electoral College
and eliminate the Senate, but no one thinks that these possibilities
are even worth discussing in that time frame.
Practical Possibility
The
notion that there are different types of possibility can be made very
precise by using the notion of a possible world. Let's try to develop
a concept of "practical possibility" that captures the claims made by
legal theorists that same legal options are not in the feasible choice
set. We can do this by narrowing the set of possible worlds that are
the domain of legal necessity in a series of steps.
Each step can be expressed in
terms of the idea of an accessibility relation. The phrase
"accessibility relation" may sound obscure, but an example will make it
crystal clear. For practical purposes, normative constitutional theory
may sometimes only be interested in those worlds that are possible
future states of the actual world. Such worlds share the history of the
actual world up to this moment, and we call worlds that have this
property "historically accessible" from the actual world. Notice that
talk about historical accessibility frequently can be translated into
talk about "path dependency." The feasible choice set–the future
choices that are possible–may be (or is) constrained by history.
But
"historical accessibility" is not a sufficient limitation for the
purposes of normative legal theory. Why not? Because it is logically
possible that the future states of the actual world could be just about
anything you can imagine; there is no logical contradiction in a
possible world that shares the history of the world up until now but in
which the United States instantly becomes a parliamentary democracy at
the next snap of Jeremy Waldron's fingers. For the purposes of
normative legal theory, we should restrict the domain of possible
worlds to those that share the basic laws of nature (physics, etc.)
with the actual world; these worlds are called "nomologically
accessible." In nomologically accessible worlds, Waldron's finger
snaps do not produce constitutional revolutions. The historically and
nomologically accessible worlds, then, are those that share the history
of the actual world up to now and that share our laws of nature.
At this point it is useful to
introduce the idea of "distance" between the actual world and some
possible world. Adjacent possible worlds are "close" to the actual
world. A possible world that was just like the actual world, except
that this entry in the Legal Theory Lexicon was never written,
would be very close, i.e., adjacent, to the actual world. Remote
worlds are "distant" from the actual world. A possible world in which
complete essays appear without effort, simply by wishing upon a star
would be more remote. Even more distant worlds are easy to imagine. In
ascending degree of remoteness, we can imagine a world in which the
Mongols conquered Europe and the Renaissance did not occur, a world in
which humans never evolved, or a world in which subtle variations in
physical laws made the evolution of carbon-based life impossible.
The
possibilities that concern normative legal theory are primarily those
that exist in historically and nomologically accessible possible worlds
that are adjacent to the actual world. One set of accessibility
relationships is especially relevant to legal discourse: these
relationships concern human psychology, institutional capacities,
social norms, and political attitudes. Some legal options will not
work, given what is true about human psychology: they make unrealistic
assumptions about what officials or citizens are capable of doing.
Other legal options would require dramatic changes in social norms:
their success relies on unrealistic assumptions about what citizens and
officials believe is acceptable or unacceptable conduct. Some options
make counterfactual assumptions about institutional capacities. And yet
other legal options are politically infeasible: they presuppose
political attitudes that only exist in possible worlds that are remote
from the actual world.
But normative legal discourse
requires what we might call "normative space." That is, normative
discourse assumes that minds can be changed and the attitudes are not
entirely fixed. We can call worlds that conform to the laws of
psychology and political science but in which attitudes and beliefs are
not fixed "psychologically and politically accessible worlds."
Practically Accessible Possible Worlds
Sometimes
normative legal theory has practical aims: it is concerned with how we
should act in the actual world. Let us call constitutional action in
the actual world "legal practice." Legal practice is not concerned
with historically and nomologically accessible possible worlds that
cannot come into being given the limits on human choices. If there is
nothing that any agent (individual, institutional, or collective) does
in any historically and nomologically accessible world that brings a
future state of the world about, then the future state is outside the
feasible choice set. Let us call the worlds that are open to human
choice in the sense just specified "practically accessible." A
practically accessible world must be nomologically and historically
accessible through human agency. Legal practice is, by stipulation,
concerned with possible worlds that are practically accessible.
Epistemological Accessibility
One
final accessibility relation requires a brief mention. We can
distinguish between those worlds that are consistent with our knowledge
of the actual world and those which are inconsistent with such
knowledge. We can use the phrase "epistemologically accessible" to
capture this idea. Worlds that are consistent with everything we know
about the actual world are epistemologically accessible. Worlds that
have a feature contradicted by our knowledge of the actual world are
epistemologically inaccessible. Not all epistemologically accessible
worlds are nomologically and historically accessible; there may be
historical facts or natural laws of which we are unaware.
The Feasible Choice Set
We
are now in a position to define the notion of a feasible choice set. A
claim that a given legal option is outside the feasible choice set is a
claim about constitutional practice, and hence a claim about which
possible worlds are practically accessible. Usually, a claim that a
given legal option is infeasible will rest (either explicitly or
implicitly) on a claim about human psychology, anthropology, economics,
sociology, or political science.
For example, the claim that a
constitutional amendment banning abortion is politically infeasible, if
fully articulated, would rest on claims: (i) about the legal
requirements for constitutional amendments, (ii) about beliefs and
desires causally relevant to the motivations of constitutional actors
such as congresspersons and state legislators, and (iii) about the
beliefs and desires of citizens. Thus, the claim might be that, given
the legal requirements, the motives of those whose assent is legally
required for a constitutional amendment, and the attitudes of voters, a
constitutional amendment banning abortion is impossible. In possible
worlds talk, we might say that worlds in which such amendments become
law are relatively remote from our own; in these worlds, political
actors behave much differently or many citizens have different
attitudes about abortion or the legal requirements for a constitutional
amendment have been altered. This remoteness is the underlying reason
for our judgment that such a constitutional amendment is outside the
feasible choice set.
Legal options that exist only
in possible worlds that are either historically or nomologically
inaccessible are outside the feasible choice set in a very strong
sense. They cannot come about in a future state of the actual world
given the natural laws that govern this world. Legal options that exist
in historically and nomologically accessible worlds may nonetheless be
only remote possibilities: they may depend on changes in beliefs,
desires, or institutions that depend on unlikely contingencies. When
such possibilities are sufficiently remote, we may say they are outside
the feasible choice set, but if we speak in this way, we are using
"feasibility" in a sense that diverges from historical and nomological
possibility.
Agent Relativity & Collective Action
One more point about the feasible choice set is that it may be "agent relative." Consider
again, the claim that a constitutional amendment banning abortion is
infeasible. When this claim is cashed out, the relevant kind of
"possibility" is likely to hinge on the assumption that the beliefs and
desires of certain agents are held constant. Given political attitudes
(as they currently exist in the actual world), such an amendment could
not be ratified by the requisite three-quarters of the fifty state
legislatures. So from the point of view of particular political actors
(e.g. an advocacy group), the constitutional-amendment option is
outside the feasible choice set. In other words, relevant to a
particular agent (e.g., an advocacy group), the feasible choice set is
constrained by the political attitudes of other agents.
But now suppose that we are
asking a different question: is a constitutional amendment possible for
the collective agent that consists of the Congress and fifty state
legislatures? From the point of view of that agent, the same legal
action (the constitutional amendment) is within the political choice
set–it is within the set of options that it is possible for that
collective agent to choose. In other words, practical possibility can
be "agent relative"–where a choice that is infeasible for one agent is
feasible for another.
I am sure that many readers of
Legal Theory Blog will recognize that this aspect of legal possibility
is related to what are called "collective action problems." For an
example, see the Legal Theory Lexicon entry on the prisoners' dilemma (Legal Theory Lexicon 007: The Prisoners' Dilemma).
Some actions require cooperation between multiple agents, but if we
make certain assumptions about the motivations of the needed
cooperators (as rational choice theory does), then cooperation and
hence the collective action may be outside the feasible choice set.
Two Mistakes About Possibility
The Mistake of Reducing Possibility to Cost Positive
and normative law and economics provide powerful frameworks for
thinking about the law and policy. Part of that power derives from a
focus on costs and benefits. Frequently, we can understand and predict
behavior by focusing on the expected costs and benefits of alternative
actions. Sometimes, the action that is morally best is the action that
accrues the greatest benefits or minimizes costs. If carried to an
extreme, this focus on costs and benefits might lead to the conclusion
that talk about possibility and feasibility can be reduced to talk
about costs. We might come to believe that the sentence, “Outcome A is
impossible,” can be reduced to, “Outcome A is too costly to be
seriously considered.” Or “Choice B is not feasible,” might be
equivalent to “Choice B is so costly that it should not be considered
as an alternative.”Possibility cannot be reduced
to cost for several reasons. First, some kinds of impossibility are
unrelated to cost (in even the broadest sense of that term). The number
of Senators apportioned to each state cannot simultaneously be equal
and proportionate to population; given unequal population size, this
would be a logical impossibility. Second, the attribution of costs to
options is dependent upon a prior notion of possibility. Thus, if we
say that option A entails the accrual of cost n, that assertion depends
on the prior assumption that a state of affairs in which option A is
chosen, but cost n is not accrued is not a possible state. Third, even
when costs play a causal role in establishing impossibility, agent
relativity differentiates between costs as factors to be weighed in
decisionmaking and costs that make certain outcomes practically
impossible. For example, if respecting the freedom of speech is costly
in cases involving advocacy of terrorism, the Supreme Court (as a
relevant constitutional agent) might take that fact into account. And
in some circumstances, high costs might lead the court to view a given
constitutional rule as “infeasible.” But this is quite different from
the court reaching the conclusion that compliance with a rule would be
impossible, because the costs associated with compliance would be too
large for the affected governmental entity to bear. In the first case,
the costs are factors to be weighed in making the decision. In the
second case, the costs are causal influences that determine which
outcomes are possible from the point of view of the Court.The Mistake of Reducing Possibility to Probability Another
tempting error is to equate possibility and probability. Possibility is
normally understood as a binary function. An action, outcome, or event
is either possible or impossible—once we specify the “sense of
possibility” or, more technically, the accessibility relationships that
define the relevant set of possible worlds. Probability, however, is a
scalar: a given outcome may be “more or less likely” or, more formally,
its probability may assume any real value from 0 to 1. The tempting
mistake is to equate “possible” with “probability greater than zero”
and “impossible” with “probability equal to zero.” Given this move, an
even more serious error becomes tempting: one might then say that
assertions of “impossibility” are actually assertions of “extreme
improbability,” with the consequence that the concept of impossibility
would turn out to be confused or mistaken. But the equation of
possibility and probability is, in fact, based on conceptual confusion.
The nature of the error is easiest to see in the case of logical
possibility. It simply does not make sense to assert that it is merely
improbable that the following two statements are both true: (1) this
article was written entirely in the twenty-first century, and (2) this
article was not written entirely in the twenty-first century. It is
logically impossible that both statements are true, and the concept of
improbability has no application to this kind of case. If someone were
to assert the following: “The probability of both statements being true
is zero,” we would understand that they were using probability in a
figurative or metaphorical sense. There may be cases where “impossible”
means “probability of zero,” but not all cases are like this. In some
cases, talk of probabilities is simply inappropriate.
Avoiding Double Standards
One
last point before we conclude. Arguments about legal possibility
should avoid double standards. That is, if one argues against a legal
alternative on the ground that it is outside the feasible choice set,
then one is obligated to show that the preferred option or options are
inside the set—on the basis of the same criteria and in light of
available evidence. The possibility of a double standard can be
illustrated by reference to a hypothetical dispute between advocates of
constitutional originalism and Dworkin’s view of law as integrity.
Originalists might claim that their approach is superior because it
provides objective standards for correct constitutional interpretation.
A Dworkinian might attempt to refute this claim by arguing actual
judges are incapable of discerning the original public meaning of the
Constitution; in the actual world, the argument might go, judges and
justices lack both the historical chops and the capacity to set aside
their own preferences. But if the Dworkinian (or quasi-Dworkinian) were
then to appeal to Dworkin’s ideal judge, Hercules, when the feasibility
of law as integrity was assessed, a double standard would have been
imposed. In the actual world, the same judges who lack historical chops
may lack Hercules’s capacity to construct the theory that best fits and
justifies the law as a whole; in the actual world, the same biases that
distort originalist judges could distort the method of law as
integrity. Of course, we can easily imagine that the tables are turned
and that it is the originalists who deploy a double standard.
Conclusion
The
concepts of "possibility" and "necessity" do a lot of work, but legal
theorists frequently make arguments about feasibility without a
theory. This leads to confusion and creates the possibility of double
standards. The aim of this entry in the Legal Theory Lexicon has been to give you the tools to think clearly and consistently about legal possibilities.
References
John Divers, Possible Worlds (Rutledge 2002).
David Lewis, On the Plurality of Worlds (new ed. Wiley-Blackwell 2001).
Lawrence B. Solum, Constitutional Possibilities, 83 Indiana Law Journal 307 (2008).
(This entry was last revised on February 15, 2009).