4 2 5S c i e n t i f i c R e v o l u t i o n sWe could have tried another solution. We could have denied the other
principle: that a sphere floats in water if and only if it is less dense than wa-
ter. Why do scientists not do this? One reason is that we have independent
evidence that water expands when it freezes. That is why jars of water burst
when they are left in a freezer. Another reason is that we could not give up
this principle alone, because it follows from more basic principles about grav-
ity and the mutual repulsion of molecules. Thus, many other areas of science
would be affected if we gave up the principle that a sphere floats in water if
and only if it is less dense than water. Scientific theories work together, so we
cannot throw out one without undermining the others. That all these other
scientific theories are not only well confirmed but useful is what makes scien-
tists give up one principle rather than another when an anomaly arises.
At this point we might seek an even deeper explanation and ask why water
expands when it freezes. In fact, to this day, nobody seems to have a fully ade-
quate explanation of this common phenomenon. There are various theories but
no agreement about how to explain the expansion of water. It has something to
do with the way in which liquid water crystallizes to form ice, but nobody is
quite sure why water crystallizes in that way. Does this show that certain phe-
nomena are beyond scientific understanding? Probably not. But it does suggest
that science may never be complete. More questions arise as science progresses,
and there will always be questions that remain unanswered. As scientists dis-
cover and explain more and more phenomena and see connections among prin-
ciples in different areas, every new step gives rise to more questions that need to
be answered. That is one way in which science makes progress.SCIENTIFIC REVOLUTIONS
Another type of scientific development is more radical—knowledge is not
simply extended, but, instead, one scientific framework is replaced (or
largely replaced) by another. What changes is not just particular claims,
but large-scale ways of doing science. In biology, the germ theory of dis-
ease and the theory of evolution through natural selection are examples of
such revolutionary developments. Einstein’s theory of relativity and the rise
of quantum mechanics were also revolutionary developments in physics.
Indeed, every branch of science has undergone at least one such revolution-
ary change during the past few centuries.
There are some important differences between scientific progress within
a framework and the replacement of one framework by another.^1 In the first
place, such changes in framework usually meet with strong resistance. A new
conceptual framework will be unfamiliar and hard to understand and may
even seem absurd or unintelligible. Even today, for example, the thought
that the Earth is spinning on its axis and revolving around the sun seems
completely counter to our commonsense view of the world. Also, arguments
on behalf of a new framework will be very different from arguments that97364_ch20_ptg01_423-448.indd 425 15/11/13 12:09 3M
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