Mike Huemer’s defense of hypothetical reasoning is so excellent that I feel like I’m cheating EconLog reader when I only quote a few sentences.  So here’s his complete discussion, featuring a shout out to David Hume qua economist.  From Huemer’s “Why I Am Not an Objectivist“:


Unfortunately, Objectivists usually object to the use of
hypothetical examples to test moral principles, on the ground that
the hypothetical examples do not represent reality. How, one might
ask, can we draw
conclusions about how the world really is from purely hypothetical
premises, i.e., premises about some imagined but not actual

This objection is vaguely felt by many who object to
thought experiments in philosophy in general, but it is a logical
error. You can validly deduce a categorical proposition from
hypothetical premises.
For example:

A -> (B -> C).

B -> not-C.

Therefore, not-A.

is a valid form of inference, where the “->” stands for the “if …
then” relation (i.e. “If x were true, y would be true”) (N.B. not
the so-called ‘material conditional’ of first-order formal logic).
And this form of inference
is relevant to the way hypotheticals are used in philosophy to test
moral principles. The typical form of thought-experiment-based
arguments in moral philosophy is as follows: “If moral theory T
were true, then
in situation S, it would be right to do A. But in situation S, it
would surely be wrong to do A. Therefore, T is false.” Notice
that this form of argument is perfectly valid: the conclusion
deductively follows from
its premises (it’s a variant on modus tollens). Notice also that
both premises are hypothetical – i.e., both are about what would be
right if so-and-so were the case. But the conclusion is

Some Objectivists refuse to even consider statements of
the form “if A then B” where A is known to be false. I’m not sure
whether they think that such statements are never true. If so,
this would be
another logical error. The proposition, “If A then B” does not
assert A. To say, “If you lose your mittens, you will get no pie,”
is not to assert that you will lose your mittens. Likewise, to
assert, “If I were a brain
in a vat, I would have no knowledge of the external world,” is not
to assert that I am a brain in a vat; it is not even to suggest
that I might be. This is obvious to anyone who understands the
English word “if”.
In general, to say, “If A were true, then . . .” does not
imply that A is true, it does not imply that A is likely
to be true, and it does not even imply that A might
true (if anything, with the use of the subjunctive mood, it
implies that A is false). For example, I can say, “If I lived in
Alaska, I would have more clothes than I presently do.” This is
true. It is true in spite
of the fact that I am not in Alaska, and I know
I am not in Alaska. When I say that, I am not implying that I
might really be in Alaska right now, and not in New Jersey as it

Some will still want to know, reasonably enough, why
thought experiments are useful. Even if they are in principle
capable of proving conclusions about actuality, why are they
Why can we not learn at least as well through the
consideration of actual or at least realistic examples? Briefly,
the reason is that hypothetical thought experiments provide a means
for conceptual
controls that often cannot be reproduced in reality. Or,
in other words, they provide a way of mentally isolating
a causal, explanatory, or logical factor for examination on its own
which normally, in the real world, cannot be isolated, and to do so
while still discussing a concrete situation.

Let me give an example to show what I mean. David Hume
once came up with this thought experiment: suppose that in the
middle of the night, the paper money in everyone’s wallet, safe, or
stash, suddenly doubled in quantity – so there is twice as much
money, but no other changes are made. Would the country then
suddenly be enormously better off – would we all be twice as
as we are now? No, in fact we would have exactly the same amount
of wealth as we presently do, for there would be exactly the same
amount of capital around, and the same availability of labor.
could then double their prices.) What this shows is that increases
in the money supply do not translate to increased wealth; it can
also be used to explain why increases in the money supply cause

Of course, such a scenario is impossible: all our money
cannot magically double in quantity. But that is not the point.
The reason the thought experiment is useful is that this way of
thinking of
it enables you to mentally isolate just the one factor
desired for consideration: the quantity of money. We imagine
just the quantity of money changed and nothing else. In
real world, one cannot do this. In the real world, it is not
possible to change the money supply uniformly (i.e.
increasing everyone’s money, without redistribution) and it is
impossible to change
the money supply without affecting the economy in some other way at
the same time. So I cannot cite a historical case in which nothing
but the money supply was altered. This is why thought experiments

A similar thing is true of thought experiments in moral
philosophy. If we want to examine the significance of one morally
relevant factor for the evaluation of actions, people, or states of
affairs, it
is useful to be able to imagine and compare cases which differ
only in respect of this one factor of interest, whereas
there may be no actual cases of which this is true.

A thought experiment, in short, is not an exercise in
fantasy but a tool of logical analysis that is necessitated by the
need for conceptual clarity (sc. distinguishing different relevant
factors from one
another in your thought), together with general facts about the
nature of reality (sc. that morally relevant, or otherwise
explantorily relevant characteristics do not come isolated in the
real world). It is a way of
concretizing abstract reasoning.

Now, if someone gives an argument against your moral
theory in which the premises are true, and the conclusion follows
logically from the premises, you can not escape from the argument
by refusing
to entertain his premises, i.e., refusing to listen to the
argument. I am going to give an argument against egoism which has
those characteristics. The premises are true hypotheticals, i.e.
true “if … then” statements,
and the conclusion that egoism is false logically follows from
them. I will not regard the mere fact that my premises are
hypothetical as showing that they must be irrelevant to (i.e. can
not entail) their conclusion,
which is categorical. I have shown above that hypothetical
premises can entail a non-hypothetical conclusion. Nor will I
regard the mere fact that the hypotheticals are counter-factual,
i.e., the antecedents are
false, and known to be so, as showing that the whole “if … then”
proposition must be false. Both of those would be gross logical
errors. If, therefore, an Objectivist wishes to answer my
argument, he will have
to do more than point out one of the aforementioned facts, and he
will have to do more than simply refuse to listen to or refuse to
think about my premises.