The Fabric of Reality David Deutch

4 
Criteria for Reality
 
The great physicist Galileo Galilei, who was arguably also the first physicist
in the modern sense, made many discoveries not only in physics itself but
also in the methodology of science. He revived the ancient idea of
expressing general theories about nature in mathematical form, and
improved upon it by developing the method of systematic experimental
testing, which characterizes science as we know it. He aptly called such
tests 
cimenti, or ‘ordeals’. He was one of the first to use telescopes to study
celestial objects, and he collected and analysed evidence for the heliocentric
theory, the theory that the Earth moves in orbit around the Sun and spins
about its own axis. He is best known for his advocacy of that theory, and for
the bitter conflict with the Church into which that advocacy brought him. In
1633 the Inquisition tried him for heresy, and forced him under the threat of
torture to kneel and read aloud a long, abject recantation saying that he
‘abjured, cursed and detested’ the heliocentric theory. (Legend has it,
probably incorrectly, that as he rose to his feet he muttered the words ‘
eppur si muove…’, meaning ‘and yet, it does move…’.) Despite his
recantation, he was convicted and sentenced to house arrest, under which
he remained for the rest of his life. Although this punishment was
comparatively lenient, it achieved its purpose handsomely. As Jacob
Bronowski put it:
The result was silence among Catholic scientists everywhere from then on
… The effect of the trial and of the imprisonment was to put a total stop to
the scientific tradition in the Mediterranean. (
The Ascent of Man, p. 218)
How could a dispute about the layout of the solar system have such far-
reaching consequences, and why did the participants pursue it so
passionately? Because the real dispute was not about whether the solar
system had one layout rather than another: it was about Galileo’s brilliant
advocacy of a new and dangerous way of thinking about reality. Not about
the existence of reality, for both Galileo and the Church believed in 
realism,
the common-sense view that an external physical universe really does exist
and does affect our senses, including senses enhanced by instruments such
as telescopes. Where Galileo differed was in his conception of the
relationship between physical reality on the one hand, and human ideas,
observations and reason on the other. He believed that the universe could
be understood in terms of universal, mathematically formulated laws, and
that reliable knowledge of these laws was accessible to human beings if they
applied his method of mathematical formulation and systematic experimental
testing. As he put it, ‘the Book of Nature is written in mathematical symbols’.
This was in conscious comparison with that other Book on which it was more
conventional to rely.
Galileo understood that if his method was indeed reliable, then wherever it
was applicable its conclusions had to be preferable to those obtained by any
other method. Therefore he insisted that scientific reasoning took
precedence not only over intuition and common sense, but also over
religious doctrine and revelation. It was specifically that idea, and not the
heliocentric theory as such, that the authorities considered dangerous. (And
they were right, for if any idea can be said to have initiated the scientific

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revolution and the Enlightenment, and to have provided the secular
foundation of modern civilization, it is that one.) It was forbidden to ‘hold or
defend’ the heliocentric theory 
as an explanation of the appearance of the
night sky. But using the heliocentric theory, writing about it, holding it ‘as a
mathematical supposition’ or defending it as a method of making predictions
were all permitted. That was why Galileo’s book 
Dialogue of the Two Chief
World Systems, which compared the heliocentric theory with the official
geocentric theory, had been cleared for printing by the Church censors. The
Pope had even acquiesced in advance to Galileo’s writing such a book
(though at the trial a misleading document was produced, claiming that
Galileo had been forbidden to discuss the issue at all).
It is an interesting historical footnote that in Galileo’s time it was not yet
indisputable that the heliocentric theory gave better predictions than the
geocentric theory. The available observations were not very accurate. 
Ad
hoc modifications had been proposed to improve the accuracy of the
geocentric theory, and it was hard to quantify the predictive powers of the
two rival theories. Furthermore, when it comes to details, there is more than
one heliocentric theory. Galileo believed that the planets move in circles,
while in fact their orbits are very nearly ellipses. So the data did not fit the