Galileo did not merely overthrow one way of looking at the solar system (the geocentric, Ptolemaic one) with a better one (the heliocentric Copernican model).  Other people had suggested that the sun was really at the center of the solar system, not the earth.  What made Galileo a truly revolutionary figure was his way of looking at the world.  He truly felt that there was no connection between the observer and the object observed.  The object observed was an independent entity, with its own history and existence.

This separation of man from the nature surrounding him was entirely new.  The man of the Middle Ages, a scant few centuries before Galileo, still felt intimately connected to many things in nature.  There were all sorts of influences and subtle correspondences that determined the medieval man’s character, his health and his moods.  All medicine of that time was based on such connections, for instance, certain planets had affinities with certain metals (iron with Mars, copper with Venus, silver with the Moon), certain plants with corresponding organs in the body and so on.  Natural disasters, like earthquakes, were seen as connected to human failings and sins.  The medieval man was totally immersed in an interacting cocoon of nature around him and in a universe around nature, consisting of a rather cozy solar system surrounded by the zodiac stars.  Man was placed right at the center of this whole structure, with everything orbiting around him on an unmoving Earth.

There were two realms of existence for the medieval man.  One was the world of nature around him, which he perceived through his senses.  The other was the divine world of the Creator.  He did not just believe in this divine world, because the Church told him to; for him, this was also a real world, which was accessible to mystics and saints directly, but which also sent down many messages and omens, especially in times of trouble, which were taken very seriously.  This divine world was taken to be located somewhere above man in the heavens.  There was also the obverse of this divine world, which was equally a direct experience to people like witches and black magicians, namely the world of the devil, located below man, at the Earth’s center.  The reality of the experience of nature was subjective, as it depended on man’s senses and presence, while the reality of the immaterial divine and infernal worlds was objective, in that it did not depend on man at all.

This entire fabric of experience and awareness of the medieval man was torn up by Galileo, though the full extent of his revolutionary thinking became evident only after his lifetime.  In fact, when he was forced to recant his beliefs in the truth of the Copernican system of planetary revolutions, it was thought by his contemporaries that all the fuss raised by him had now subsided and things were back in their accustomed order, as mandated by the Church.  But Galileo was the first to articulate a new consciousness of the world in a new age that was dawning, so that later thinkers did not treat him as a unique oddity but rather as a pioneer, who had suffered for his prescient knowledge and convictions.  It took only a little more than half a century after Galileo’s death for Newton to formalize Galileo’s vision of the world in his monumental synthethis of natural laws, the Principia Mathematica.  Newton ruled physics for over three hundred years and the secular Age of Reason, based on the new science, did away with the last vestiges of medievalism.

This is the background of Galileo’s achievement.  What exactly did he do that was so extraordinary?  We have said that he no longer recognized any connection between the observer and the object observed.  That meant two things:  he ripped man out of his entire, cozy medieval cocoon of connections, relationships and correspondences with the natural world.  Man, the observer, was now completely independent of the observed world, as this world was of him.  But is also meant that Galileo eliminated with one chop of the axe the reality of the entire immaterial worlds of God and of the devil: both these worlds were now relegated to realms of belief only and (for many) to mere superstition.

As an educated man of his time, Galileo was well aware of the Greek philosophical traditions that had not been challenged for over two thousand years and to which the Church in his own time still adhered.  For Plato and Aristotle, the only world containing permanent values and true knowledge was the upper world of the divine.  This world, by definition beyond the ordinary human senses, was a real world, not one of belief only.  “Contemplation” of it was the only way to real knowledge.  In contrast, the knowledge to be gained on earth was of a very inferior kind, transitory, evanescent and unsatisfactory, because of the unreliability of the human senses, which depended on many purely personal factors.  There was an intermediate stage of knowledge, which Plato said could be acquired by a study of ‘geometry’, or what we would call mathematics today.  He called it a ‘bastard’ science, because it was still only a product of the human mind and thus represented no true knowledge.  It could always be overturned by a better argument, based on (human) logic.  Nevertheless, Plato encouraged his students to study mathematics, because it did raise thoughts beyond the gross material level.

In this Greek system, the transitory reality of the natural world was subjective, because it depended on the human senses and the human presence.  The reality of the divine world, however, was objective, because it was quite indenendent of man and his senses, or of any other created beings.

Galileo was convinced that his new way of looking at the world was capable of unlocking the secrets of nature through a process of careful observation and rational analysis.  His work on motion convinced him, as he put it, that “the language of nature was mathematical” and that without mathematics we could not understand a single word of it.  It followed, therefore, that any hypothesis about natural processes that was repeatedly confirmed by observation and experiment, would cease being a mere hypothesis and become a truth about the laws of nature, something established permanently by science.

Galileo agreed with the Greeks that most of what we perceived through our senses was purely subjective, therefore unreliable and totally unsuited for any scientific treatment.  He stated very clearly that anything perceived by the organs of touch, smell, taste and hearing fell into this category, which he called the “secondary qualities”.  He also felt, however, that the absence of these sense organs would leave a small area which did not depend on them or our presence.  He called these the “primary qualities”.  They were very few in number and Rene Descartes later reduced them to just two, matter and motion.  These two “qualities” did not depend on our senses and were thus fit to be the subject of scientific study.  Because of their independent status, Galileo postulated that they, and they alone, possessed objective reality.  

As time went on, this objective reality was extended to all perceived phenomena of nature, not just matter and motion, so that by the end of the nineteenth century the only reality recognized by science was the independent existence of the natural world.  For things to be ‘real’ they had to be material and this reality did not depend on man, his senses or his presence.

These comforting certainties were rudely shaken up in the early twentieth century when physics breached the atomic barrier and gave birth to relativity and quantum mechanics.  The limitations of Newton’s physics were exposed.  It no longer represented the undisputedd truth of nature’s laws, but only a good, first approximation of the facts.  Therefore hypotheses now remained hypotheses only, no matter how many times experiments showed their correctness.  In this, modern physics agreed with the Greek traditions of Plato and not with Galileo.  Also, mathematics was now recognized as proceeding only from the human brain: its arguments could be overturned by better logic at any time and thus did not correspond to any independently existing outside facts.  The seminal work done by Kurt Goedel in mathematical logic was pivotal in this area.  Again, this merely stated in modern language what Plato had said about ‘geometry’.

Finally, the latest string theories in physics call for a one-dimensional particle which, as such, cannot exist in out world.  It is however defined as the ultimate, irreducible matter particle, so it must be “real” in some way, if matter is also “real” in some way.  The reality of matter is recognized by modern physics to be subjective, because its perception depends on our senses and thus our presence.  As the one-dimensional string particle cannot be perceived by our senses in this manner, what kind of ‘reality’ can it possess?  The only other kind of reality that we can conceive of is the objective kind, which does not depend on our senses.  It seems, therefore, that modern physics is going to need to reintroduce objective reality into its thinking.  Galileo tried to keep objective reality in his new scientific method, by making matter and motion objectively real, but this failed when it was realized that everything in nature that we perceive can only be subjective.  Therefore anything objectively real, like the string particle for instance, can exist only in an immaterial world, beyond our senses.

The above line of argument shows that modern physics agrees very closely with all the Greek traditions that Galileo strove so hard to overthrow in the seventeenth century.  This is especially important when it comes to Galileo’s treatment of objective reality.  This has no place in our world of subjectively perceived nature, where Galileo tried to make use of it.  It does, however, have a place in an expanded realm of physics, where both the subjective world of material nature and an objectively real but immaterial world are included in its conceptual structure.