Below are some of the reasons why this is true.
A theory can be a mixture of true and false assumptions, and correct predictions can be made by the true assumptions.
Theories are often a mixture of correct assumptions and mistaken assumptions. Correct assumptions in a theory may imply certain predictions, which may prove successful. But the theory may still contain incorrect assumptions, which did not imply the predictions that turned out true. The correct predictions only tend to confirm (perhaps to at least some degree) those parts of a theory that implied those correct predictions, not other assumptions that did not imply those predictions.
For example, some people advanced the theory in 2002 that the Bush administration had secretly orchestrated the September 11 attacks, to create a pretext for war because it wanted to invade Iraq. Perhaps some of those people then said in 2003 that their theory was confirmed, because the Bush administration really did invade Iraq in that year. But in this case we have a theory making two assumptions: (1) the assumption that the Bush administration orchestrated the September 11 attacks; (2) the assumption that the Bush administration wanted to attack Iraq. The invasion of Iraq in 2003 may tend to confirm the second of these assumptions, but not the first.
So when a particular scientific theory seems to be confirmed by some prediction that eventually matches observations, we need to ask: which parts of the theory tend to predict the prediction that matched observations? Only such parts – if any-- should be considered as having been put (possibly) in a favorable light by the observations.
Multiple theories may make a particular prediction, so a confirmation of the prediction may not really support a particular theory that makes that prediction.
It is not necessarily true that a
confirmed prediction tends to show that the theory that predicted it
is true, because there may be many other reasonable theories that
make the same prediction. For example, let's imagine a person in 2007
arguing that sinister forces on Wall Street were trying to
orchestrate a sharp economic downturn, so that they could make lots
of money on certain types of stock market bets called puts (which
increase in value when a stock goes down). In 2008 (when such an
economic downturn occurred) such a person would no doubt say, “Look,
we did have a sharp economic downturn; my theory is
confirmed.” But such reasoning would be invalid, because the same
sharp economic downturn was predicted by various other theories, such
as the theory that a housing bubble would produce such an economic
downturn, and the theory that too much consumer credit would produce
an economic downturn.
The favorite device of a theory
advocate is a “predicted versus actual” line graph. Here is a
very simple example of this type of graph, with the blue line showing
predicted results and the red line showing observed results:
This type of graph can be used to
try to show that a particular theory is matching observations. But be
distrustful when you see such a graph. Why? Because it is easy to
cherry-pick either the data used as the “observed data” or the
data used as the “predicted data,” or both.
This is particularly true in any case
where the data points are not some simple thing (depending on one
observation, as in the case above), but instead require some
complicated summary of multiple observations. In such cases it is all
too easy for a presenter to massage the data in a way that shows a
theory in a favorable light. Given a choice of five different ways of
showing the “observed results” (each using a different source of
data, or a different way of summarizing the data), someone can choose
whichever set of “observed results” is most in agreement with his
theory.
Another way in which bias can be
displayed is by massaging and cherry-picking the “predicted results”
shown in a graph such as the one below. Theories often have multiple
flavors, which vary because of a choice of parameters that can be
used within the theory. In other words, the “predicted results”
from a particular theory are often very fuzzy, rather like an
electron probability cloud. A presenter can pick particular values
within that fuzzy cloud that most closely match the “observed
results,” and plot such values as the “predicted values” on a
line graph. The result will show the theory in the most favorable
light, but may be misleading. For a recent specific example of this type of
cherry-picking, see this blog post.
Another way in which bias can be shown
in matching observed results with predicted results is simply by
choosing the start point and the end point of the data being graphed.
For example, if I have a theory that bonds tend to out-perform
stocks, I may use a start point of January 1, 2000 and an end point
of Dec 1, 2008. That will show a huge advantage for investing in
bonds as compared to investing in stocks. But a different start point
and end point would tell a very different story. A similar technique
can be used to try to show the likelihood of a particular scientific
theory. A supporter can choose to graph whatever start point and end
point shows the closest match between the theory and observations,
even though different start points and end points on the line graph
would show a much smaller degree of agreement.
One type of reasoning sometimes made
is: x is the only theory that predicts the observed phenomenon
y, so x must be true. But that does not follow. The
human imagination is weak, and our ignorance is enormous. Almost any
observed phenomenon can be explained in many different ways, but the
puny human imagination may be able to think of only one or two of
those ways. Back during the days of the Black Plague, the theory of
“God's wrath” may have been the only theory that explained why so
many people were dying, but it would have been wrong at that time to
assume such a theory was correct on that basis.
With sufficient ingenuity, unbelievable theories can be contrived to make predictions that match observations.
Sometimes it is possible for a theory
to make some correct predictions, even though the theory isn't
plausible. One of the most famous examples is the Ptolemaic theory, a
theory of the solar system. The theory held that the Earth was at the
center of the solar system. To make such a theory match observations,
the theory included a complex model of planetary motions, in which
planets orbited in small orbits called epicycles that were part of
much larger orbits. The predictions of the Ptolemaic theory seemed
accurate for centuries, but the theory was quite false.
There are modern-day equivalents of the Ptolemaic theory -- theories that are very suspect because of their excessive complexity and contrivance.
There are modern-day equivalents of the Ptolemaic theory -- theories that are very suspect because of their excessive complexity and contrivance.
Implausible, contrived scientific theories are like this
Scientific theories are only well-confirmed by predictions when
the theories make very many predictions that have been confirmed by
observations.
Thinking that a scientific theory has
been confirmed because it makes a few correct predictions is like
thinking that you've proven you're a great baseball player because
you've pounded out a few base hits. You've only proven yourself a
great baseball player if you've made hundreds or thousands of hits.
Similarly, the only scientific theories that are well-confirmed by
predictions are those that have made hundreds, thousands or millions of
predictions that have been confirmed.
We have a great example of such a
theory: the theory of gravitation. The theory is based on a simple
exact formula that you can use to compute the degree to which massive
bodies attract each other. Scientists and engineers (and the
computers on spacecraft) have used this theory thousands or millions
of times, and the predictions made by the theory have always proven
true. A robot spacecraft could never reach Mars and land on Mars
unless the predictions of the theory of gravitation proved true
thousands of times, nor could the Apollo astronauts have landed on
the moon and returned.
Another such theory is the theory of
electromagnetism. The theory is based on a simple exact formula you
can use to compute the attraction between two electrical charges.
Scientists and engineers have used the formula thousands or millions
of times, and it always gives the right answer.
Compared to these theories, any theory
that claims scientific validation because it seems to make a few
correct predictions is like some kid who claims to be a professional
actor because he acted in a few high-school plays.
According to the standard I mention
here, we might have a reason for regarding many well-known scientific
theories as being on rather shaky ground, as they do not make a huge
number of predictions that have been confirmed. For example, we
might regard as a very shaky theory the theory that life first arose
on planet Earth merely because of a lucky chance combination of chemicals. Such a
theory does not make a huge number of predictions that have been
confirmed, and in fact, does not seem to make any prediction that has
been confirmed.
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