This short comment on Epstein's (2008) paper and on the response by Thompson and Derr argues that the symmetry between explanation and prediction cannot satisfactorily be discussed without making clear what prediction means - depending on which connotations the authors have with 'prediction' their arguments can or cannot be accepted.[No keywords]

Klaus G. Troitzsch

English

Research Papers in Economics

©Copyright JASSS
 
Klaus G. Troitzsch (2009)
Not All Explanations Predict Satisfactorily, and Not All Good
Predictions Explain
Journal of Artificial Societies and Social Simulation vol. 12, no. 1 10
<http://jasss.soc.surrey.ac.uk/12/1/10.html>
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Received: 07-Jan-2009    Accepted: 07-Jan-2009    Published: 31-Jan-2009
Abstract
This short comment on Epstein's (2008) paper and on the response by Thompson and Derr
(2008) argues that the symmetry between explanation and prediction cannot satisfactorily be
discussed without making clear what prediction means—depending on which connotations
the authors have with 'prediction' their arguments can or cannot be accepted.
1.1
Both Epstein (2008) and the authors of response (Thompson and Derr 2008, this issue of
JASSS) miss one important point: Both take "prediction" as a term with a fixed and
unambiguous meaning—which it is not. Epstein seems to understand prediction in the sense
this word has in the sentence "the orbits of the planets will never be predicted." By the way, in
this argumentation he forgets that 2000 years before Newton solar and lunar eclipses could
be predicted with surprisingly high precision with a model which we now find wrong (which
means that a good prediction is not necessarily a good explanation). Thompson's and Derr's
reply accepts a much weaker meaning of "predict" as they call "earthquakes occur" a
prediction (for which no theory is necessary, after the first earthquake that ever occurred it
was clear to humankind that such things happen). Of course, the predictive power of plate
tectonics is even higher as it predicts that in certain parts of the planet earthquakes will be
more frequent than in other regions. This finding makes it necessary to argue that there are
at least three different meanings of prediction, namely as an answer to three quite different
questions (see Troitzsch 1997, p. 46):
1
"Which kinds of behaviour can be expected [from a system like this] under arbitrarily
given parameter combinations and initial conditions?
2
Which kind of behaviour will a given target system (whose parameters and previous
states may or may not have been precisely measured) display in the near future?
3
Which state will the target system reach in the near future, again given parameters and
previous states which may or may not have been precisely measured?"1.2
With this distinction between different kinds of prediction, the controversy between Epstein
and his critics is easily ended: Any kind of good explanation, under all circumstances will
yield a prediction of type 1 (and perhaps also of type 2), but not every good explanation will
yield a prediction if type 3. The prediction of type 3 can even be subdivided in a stochastic or
statistical version and a deterministic one (see Troitzsch 1997, p. 46-47):
3a
Which are the expected value and the confidence interval around the expected value of
the state the target system will reach in the near future, again given parameters and
previous states which may or may not have been precisely measured?
3b
Which exact value will the state of the target system have at a certain point of time in
the near future, again given parameters and previous states which have been precisely
measured? (This version was not even mentioned in Troitzsch 1997 and seems to be
excluded in Epstein's paper, paragraph 1.8, as well!)
The prediction of a solar or lunar eclipse is obviously of type 3b, as everybody knows who
ever observed one of these phenomena. The prediction of the temperature at some time
during the next few days is obviously of type 3a, as we know from weather forecasts on the
TV (where they usually show the trajectory of the expected temperature for the next few days,
surrounded by a funnel which shows the growing confidence interval).
1.3
But there is another claim to make: it is not the case that Carl Hempel's symmetry hypothesis
was never contested. Some fifty years ago there was a long discussion (see Scriven 1959 and
Grünbaum 1962, and see also Stegmüller 1966, where he points to the vagueness and
ambiguity of the terms explanation and prediction, and Stegmüller 1969, chapter II, with a
very detailed analysis of explanation, retrodiction and prediction). Hempel's argumentation
with respect to the symmetry thesis after this discussion (Hempel 1965) is much more
detailed, and he argues only that an adequate explanation is a potential prediction (a thesis
which he defends strongly) and that an adequate prediction is a potential explanation (here
he is more retentive, in principle accepting that the precise predictions of solar eclipses by
ancient astronomers were based on wrong explanations). The problem here is the word
"adequate". It is quite obvious that his notion of adequacy is similar to Zeigler's (1985, p. 5)
"structural validity" since whenever a model is structurally valid ("if it not only reproduces the
observed real system behaviour, but truly reflects the way in which the real system operates
to produce its behaviour") it can be expected to make predictions of any of the types 1
through 3a (of type 3b only in the entirely deterministic case)—but even non-adequate
explanations are potential and even good predictions: see the case of solar and lunar eclipses
predicted by Babylonian astronomers.
1.4
In the case of living, cognitive and social systems (and in other complex systems as well)
cause-effect relations are rarely as simple as in the examples referred to by Hempel,
Grünbaum, Scriven and Stegmüller mentioned above (their arguments are mainly about
artillery, astronomy, hydraulics, only Scriven refers to medicine, namely the case of late
symptoms of a particular disease which may or may not occur—but Scriven is rather an
opponent of the symmetry thesis). In these complex systems sometimes the same effect can
be produced by different combinations of different causes (see the discussion in Hempel
1965, footnote 46, where he argues that in the medical cases referred to by opponents of the
symmetry thesis the available information is often not sufficient to explain the observed
outcome). And this is in line with Epstein's argumentation that "Explanation Does Not Imply
Prediction"—if and only if we interpret Epstein's prediction as a prediction of type 3b; if we
restrict the meaning of prediction to type-1 or type-2 prediction, then a claim that an
explanatory theory can predict is certainly possible.
1.5If we have a closer look at Thompson's and Derr's argumentation about earthquakes (and the
same applies to Epstein's two other examples and the examples discussed between Scriven,
Grünbaum, Hempel, Stegmüller and others fifty years ago), we will see that a potential
deficiency of both explanation and prediction is not (only) a deficiency of the respective
theories but rather a deficiency of our knowledge of the complicated antecedents of the
events to be explained or predicted: That we "can neither explain nor predict that an
earthquake of specific power will occur in a specific place at a specific time" is mainly because
we do not have sufficient information about the exact shape of the neighbouring plates, their
speed, their temperature, the pressure that they exert on each other, their viscosity and so
forth. If we had, it would still be impossible to make a prediction of type 3b, but a prediction
of type 3a might be more precise and thus more valuable, but it is very doubtful that all these
characteristics of some matter several kilometres below our feet will ever be measurable to
any useful degree of precision. And in a way this resembles the situation of those who want to
explain why a crash in the stock market occurred or who want to predict when such a crash
will occur. In agent-based simulation of a stock market a crash will easily be predicted (as the
simulation proceeds) and will easily be explained (as the computer simulation makes all data
available, as we can "look into the minds" of our software agents), but even if the model were
structurally valid in Zeigler's sense we have no chance to initialise the simulation model with
an exact image of all the knowledge, beliefs, desires and intentions of all the brokers,
shareholders, buyers and sellers involved in a stock market, and we would arrive at a
prediction of type 2 at best.
1.6
But this problem, too, was already seen by Hempel (1965) when he commented on Scriven's
argument. Hempel concedes that sometimes only the occurrence of the event to be explained
reveals that the conditions laid down in the explanans were fulfilled. Thus if an event is to be
predicted we still lack the information whether (all of) the antecedent conditions are fulfilled.
From Hempel's point of view this is rather a practical than a logical problem, as in principle
the necessary information could be provided. Thus in the end the controversy might be
settled by summarising that the symmetry thesis holds from a logical point of view but not
from a practical point of view.
 References
EPSTEIN, Joshua M. (2008) Why Model? Journal of Artificial Societies and Social Simulation vol.
11, no. 4 12 http://jasss.soc.surrey.ac.uk/11/4/12.html
GRÜNBAUM, A. (1962) Temporally-asymmetric principles, parity between explanation and
prediction, and mechanism and teleology. Philosophy of Science, 29: 162-170, partly
reprinted in L. I. Krimerman (ed.), The Nature and Scope of Social Science. A Critical
Anthology, Appleton-Century-Crofts, New York, NY. 1969, pp.126-132.
HEMPEL, C.G. (1965) Aspects of Scientific Explanation and other Essays in the Philosophy of
Science. New York: The Free Press
SCRIVEN, M. (1959) Explanation and prediction as non-symmetrical. Explanation and
prediction in evolutionary theory. In L. I. Krimerman (ed.), The Nature and Scope of Social
Science. A Critical Anthology, Appleton-Century-Crofts, New York, NY. 1969, pp. 117-125.
First published in 1959 in Science 130: 477-482.
STEGMÜLLER, Wolfgang (1966) Erklärung, Voraussage, wissenschaftliche Systematisierung
und nicht-erklärende Information. Ratio 8, S. 1-22
STEGMÜLLER, Wolfgang (1969) Probleme und Resultate der Wissenschaftstheorie und
Analytischen Philosophie. Band I: Wissenschaftliche Erklärung und Begründung, Berlin:
Springer 2nd ed. 1983 (first edition 1969)THOMPSON, N. S. and DERR, P. G. (2009) Contra Epstein, good explanations predict. Journal
of Artificial Societies and Social Simulation vol. 12, no. 1 9
http://jasss.soc.surrey.ac.uk/12/1/9.html
TROITZSCH, Klaus G. (1997) Social Science Simulation—Origins, Prospects, Purposes. In:
Rosaria Conte, Rainer Hegselmann, Pietro Terna, eds.: Simulating Social Phenomena, Berlin
etc. (Springer) (LNEMS 456), pp. 41-54
ZEIGLER, B.P. (1985) Theory of Modelling and Simulation. Krieger, Malabar.
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Aspects of Scientific Explanation and other Essays in the Philosophy of Science.

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Why Model?

Not All Explanations Predict Satisfactorily, and Not All Good Predictions Explain

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Not All Explanations Predict Satisfactorily, and Not All Good Predictions Explain

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