Nicholas Copernicus (1473-1543)

The Heliocentric SystemIn On the Revolutions of the Heavenly Bodies (published as Copernicus lay on his deathbed), Copernicus proposed that the Sun, not the Earth, was the center of the Solar System. Such a model is called a heliocentric system.

The ordering of the planets known to Copernicus in this new system is illustrated in the following figure, which we recognize as the modern ordering of those planets.

The Copernican Universe

In this new ordering the Earth is just another planet (the third outward from the Sun), and the Moon is in orbit around the Earth, not the Sun. The stars are distant objects that do not revolve around the Sun. Instead, the Earth is assumed to rotate once in 24 hours, causing the stars to appear to revolve around the Earth in the opposite direction.

The heliocentric hypothesis was rejected out of hand by virtually all.

Copernicus was an unlikely revolutionary. It is believed by many that his book was only published at the end of his life because he feared ridicule and disfavor: by his peers and by the Church, which had elevated the ideas of Aristotle to the level of religious dogma. However, this reluctant revolutionary set in motion a chain of events that would eventually produce the greatest revolution in thinking that Western civilization has seen.

His ideas remained rather obscure for about 100 years after his death. But in the 17th century the work of Kepler, Galileo, and Newton would build on the heliocentric universe of Copernicus and produce the revolution that would sweep away completely the ideas of Aristotle and replace them with the modern view of astronomy and natural science. This sequence is commonly called the Copernican Revolution.

Kepler was forced finally to the realization that the orbits of the planets were not the circles demanded by Aristotle and assumed implicitly by Copernicus, but were instead the "flattened circles" that geometers call ellipses.

Galileo Galilei: 1564-1624

Galileo made extensive contributions to our understanding of the laws governing the motion of objects.

Galileo did not invent the telescope (Dutch spectacle makers receive that credit), but he was the first to use it to study the heavens systematically. His little telescope was poorer than even a cheap modern amateur telescope, but what he observed in the heavens rocked the foundations of Aristotle's universe and the theological-philosophical worldview that it supported.

It is said that what Galileo saw was so disturbing for some officials of the Church that they refused to even look through his telescope; they reasoned that the Devil was capable of making anything appear in the telescope, so it was best not to look through it.

Galileo observed the Sun through his telescope and saw that it had dark patches that we now call sunspots (he eventually went blind, perhaps from damage suffered by looking at the Sun with his telescope). Furthermore, he observed motion of the sunspots indicating that the Sun was rotating on an axis.

These "blemishes" on the Sun were contrary to the doctrine of an unchanging perfect substance in the heavens, and the rotation of the Sun made it less strange that the Earth might rotate on an axis too, as required in the Copernican model. Both represented new facts that were unknown to Aristotle and Ptolemy.

The famous Leaning Tower of Pisa experiment may be apocryphal.

It is unlikely that Galileo himself dropped two objects of very different weight from the tower to prove that they would hit the ground at the same time. However, it is certain that Galileo understood the principle involved, and probably did similar experiments.

The realization that, as we would say in modern terms, the acceleration due to gravity is independent of the weight of an object was important to the formulation of a theory of gravitation by Newton.

In popular lore, Galileo Galilei (1564–1642) dramatically refuted Aristotle’s laws of motion by dropping unequal weights from the Leaning Tower. In the scientist’s writings, however, he never claimed to have conducted an experiment from that tower. Instead, his first biographer, Vincenzo Viviani, launched the story roughly a dozen years after the great man’s death.

Galileo's challenge of the Church's authority through his assault on the Aristotelian conception of the Universe eventually got him into deep trouble with the Inquisition.

Late in his life he was forced to recant publicly his Copernican views and spent his last years essentially under house arrest. His story certainly constitutes one of the sadder examples of the conflict between the scientific method and "science" based on unquestioned authority.

Isaac Newton: 1642-1727

The legend is that Newton saw an apple fall in his garden, thought of it in terms of an attractive gravitational force towards the earth, and realized the same force might extend as far as the moon.

If I have seen further it is by standing on the shoulders of giants.

Isaac Newton, Letter to Robert Hooke, February 5, 1675

Newton's First Law of Motion:

I. Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.

Newton's Second Law of Motion:II. The relationship between an object's mass m, its acceleration a, and the applied force F is F = ma. Acceleration and force are vectors (as indicated by their symbols being displayed in slant bold font); in this law the direction of the force vector is the same as the direction of the acceleration vector.

Newton's Third Law of Motion:

III. For every action there is an equal and opposite reaction.

Alexander Pope on Isaac Newton

"Nature and nature's laws lay hid in night; God said Let Newton be! and all was light."

Francis Bacon: 1561-1626

Bacon's Philosophy:Francis Bacon's major contribution to philosophy was his application of induction, rather than the a priori method of medieval scholasticism.

"There are and can be only two ways of searching into and discovering truth. The one flies from the senses and particulars to the most general axioms: this way is now in fashion. The other derives axioms from the senses and particulars, rising by a gradual and unbroken ascent, so that it arrives at the most general axioms last of all. This is the true way, but as yet untried."

-Francis Bacon

René Descartes (1596 - 1650)

When studying human consciousness Descartes concluded that the human mind was a substance distinct from the physical brain. However he also believed that they were linked. The link between the brain and the mind, according to Descartes, was the immaterial mind or the soul. Descartes believed that the soul was a feature unique to humans who have the ability for sophisticated social interaction - i.e., language.

Descartes was of the belief that science should be grounded, not in observation and prediction, as Bacon thought, in absolute certainty. He used three principles to describe his philosophy.

1. To employ the procedure of complete and doubt to eliminate every belief that does not pass the test of indubitability (skepticism);

2. To accept no idea as certain that is not clear, distinct, and free of contradiction (mathematicism).

3. To found all knowledge upon the bedrock certainty of self-consciousness, so that "I think, therefore I am" becomes the only innate idea unshakable by doubt (subjectivism).

The existence of God is the first and the most eternal of all truths that exist and the only one from which proceed all others.

Descartes

It is quite evident that existence can no more be separated from the essence of God than the fact that its three angles equal two right angles can be separated from the essence of a triangle … It is … a contradiction to think of God (that is, a supremely perfect being) lacking existence (that is, lacking a perfection).’

In other words, existence is a property no perfect being could be without out, and so a perfect being must exist. Critics claim that Descartes has effectively ‘defined God into existence’.

dualism, Cartesian interactionist - The view that: (1) the mental and the material comprise two different classes of substance and; (2) each can have causal effects on the other.