Peter Bennett An Essay concerning Personal Identity 20
3 A particular type of counterfactual situation is discussed in the
philosophy of personal identity. This section aims at describing what
this situation is.
According to Wilkes, thought experiments are characterized
firstly by being imaginary - a thought experiment cannot be realized
in this world - and secondly by observing the constraints by which
actual experiments are constrained. There is a problem here since
one constraint on a real experiment is that it is actually carried out.
To get round this let us say that the second requirement covers
what is called 'experimental design' rather than the actual execution
of the experiment. (The problem of not actually being able to do
your experiment will be discussed shortly.) One constraint which will
be important to my argument is that thought experiments, just as
much as real experiments, always have an aim of confirming or
rejecting some hypothesis. Typically there is something you control -
the dependent variable - and something you measure - the independent
variable. One way for real experiments to go wrong is for an
uncontrolled variable to creep in. This claim is often made against a
thought experiment. For example someone who believes that
continuity of character is important for personal identity might argue
against a brain transplant argument by saying that the individuals
coming out of a brain transplant must have characters radically
different to those of the individuals going in. Anybody who tries to
ask 'Which is Jack and which is Jill?' after the experiment, which
has not been controlled for character, has to deal with the fact
that as far as the character-theorist is concerned, the answer
is neither.
Bernard Williams's solution to the problem posed by TE2 and
TE3 is similar. In TE1 Locke was very quick to conclude that
psychological continuity was what was important for personal
identity. (The next chapter deals with Locke in greater detail and
precision.) But we could say that he failed to control for the fact
that bodily continuity might be part of _
Peter Bennett An Essay concerning Personal Identity 21
what is involved. TE2 and TE3 bring this out. Williams has
concluded elsewhere11 that bodily continuity must be part of what is
involved in personal identity. Such a conclusion could be used to
solve the problem presented by TE2 and TE3. But this is not the
only conclusion that one can draw from these thought experiments. I
shall eventually argue something different.
But this chapter is concerned with the methodology of thought
experiments, and as we have just seen the first question to ask
about a thought experiment is:
Q1: Does the putative thought experiment satisfy the
constraints required of real experiments?
There are two kinds of 'experiment' which Wilkes excludes from
further discussion. The first are those which 'take place in thought'.
Wilkes gives the example of mentally parsing the sentence 'colourless
green thoughts sleep furiously' to see if it is grammatically correct.
One could also put Descartes's cogito and William James's
introspection under this heading. Such activities have peculiar
problems of their own, but like Wilkes, I want to exclude them from
further discussion, since these cases are not like the things which go
on in the philosophical discussion of personal identity. I shall
therefore record the exclusion of this kind of thought experiment as
our second question to ask about thought experiments:
Q2: Does this thought experiment merely take place
in thought?
_
11 'Personal identity and individuation', also in Problems of the
Self, pp. 1-18. The purpose of 'The Self and the Future' is
more to present a puzzle than to offer a solution. My solution
will be offered in the conclusion to this thesis.
Peter Bennett An Essay concerning Personal Identity 22
The second kind of thought experiment which Wilkes excludes
from the discussion are those which are 'merely imagined', i.e. could
have been done but were not. Wilkes gives the example that it is
likely that Galileo merely imagined what would happen if he dropped a
cannon ball and a musket ball simultaneously, rather than actually
doing it. If this is the case then Galileo might be accused of sloppy
science for not actually trying it out, but the imaginary scenario is
not a thought experiment in the sense which Wilkes wants
to discuss.12
If a thought experiment is not 'merely imagined' then there is at
least a doubt as to whether the hypothetical situation could come
about in this world. I shall call such impossible thought experiments,
which do not merely 'take place in thought' and are not 'merely
imagined', fantastic thought experiments. This terminology, if mildly
abusive, is used in the same kind of spirit as is talk of 'mad
scientists'. The terminology will alert us to the fact that we are
moving outside the circle of experience, and so 'can be sure of not
being contradicted by experience'.13
But already there may be cause for caution, since in real
experiments it often happens that things do not work out in quite the
way that was expected. Indeed there would be little point in doing
the experiment if we could be absolutely certain of the outcome.
There may be unforeseen deep impossibilities in the experiment. (I
shall say what a deep impossibility is shortly.) So the third question
to ask about a thought experiment is:
Q3: Is the thought experiment merely imagined?
_
12 For more details about Galileo's thought experiment and further
examples of thought experiments from physics, see J. R. Brown,
'Thought experiments since the Scientific Revolution'. Wilkes
mentions Galileo's thought experiment on p. 3 of Real People.
13 I. Kant, Critique of Pure Reason A4/B8.
Peter Bennett An Essay concerning Personal Identity 23
The fourth question to ask is:
Q4: Is the thought experiment merely illustrative?
Consider:
TE4: Travelling at the speed of light
Einstein, contemplating Maxwell's electrodynamics,
wondered what someone travelling beside the front of a
beam of light would see. No individual could travel at
this speed - the experiment could not be performed. But
if he did, then according to Maxwell's theory, he should
see something that could not exist: a stationary
oscillatory field. Hence, from this thought experiment
alone, there are at least serious difficulties with
Maxwell's theory. The imaginary state of affairs is an
individual travelling at the speed of light; the implication
drawn, that the theory is inconsistent.14
Wilkes portrays this as an example of a good scientific thought
experiment and Parfit describes it as 'worthwhile',15 which in a way
it is. But one must not suggest that this thought experiment by
itself brought about the downfall of Maxwell's electrodynamics. To
do so would be to promulgate an implausible myth. What the thought
experiment did do was suggest a line of research which turned out
to be fruitful. Such thought experiments are useful to science as
informal reasoning is useful in mathematics. But at the end of the
day these informal techniques need to be backed up by rigorous
methods, and this cannot always be done using thought experiments.
_
14 Real People pp. 3-4.
15 Reasons and Persons p. 219.
next
back
contents
Copyright Peter Bennett 1992