Aristotle and Greek Thought: Understanding the Influence on Physics

1. Aristotle (& Greek thought): Because the ideas of the Greeks of antiquity concerning much of physics is essentially wrong according to our modern understanding, we don’t want to spend too much time on them, but it is worthwhile understanding why it was natural for them to hold their views. We have to be careful, and often skeptical, of our own views in physics, even today - especially with what has happened in the last century (20th century) in physics. We will see that we often have to make major adjustments in our views of what constitutes the basic laws of nature, and these laws and “rules” often do not jive with our intuitive everyday experiences. We have to get used to new “not-so-intuitive” ideas. In addition, the views of Aristotle were so influential that they more or less controlled all intellectual activities in what we might refer to as physics for 2000 years, and as a result hindered its development. He was quite an influential character.

2. Imaginative Background To comprehend the views of the Greeks concerning physics (or of any age, for that matter), we must understand their imaginative background (our world paradigm). In addition to our formal backgrounds (usually acquired by education), every one of us has a much simpler interpretation of the world, which we are often unaware of and which will affect the way we try to understand our physical surroundings. We may even unconsciously conceal this view (even from ourselves), but it takes training to achieve a sophistication, and objectiveness, in order to deal intuitively and effectively with many of the more abstract (formal) concepts of physical theory. The imaginative background of Aristotle was quite different than our imaginative background nowadays. Why is this? Well, the environment was very different.

3. Purposeful motivation For example, there were no machines present (at least not in our present understanding), and locomotion was achieved on foot, or by animals (and slaves performed the laborious duties that machines do today); that is, it always included some purposeful motivation, usually of a living being. It was not easy to imagine flying objects (planes) or moving trains which are propelled by physical principles, where is easy to separate our thoughts from any purposeful implementation of the motion. The flying objects were birds, and they could easily be associated with magical properties, and a purpose. Our environment today is full of examples in our material environment of machines and objects that seem to be propelled by lifeless technology. Nowadays we do not image that there is a horse in our car in spite of our measure of power - horse power.

4. What influence does this have on the conceptual imaginativeness of the observer, even an interested “scientist” (we use the word in a broad sense)? There are two types of motion important for the Greeks of antiquity: (1) the movement of animals, and (2) the movement of the heavenly bodies. To the Greeks, except for a few (e.g. Democritus and Archimedes), it was not natural for them to imagine a purely mechanical view of the world when considering motion. We can see the analogy between animals and machines, but to the Greeks, the peculiarity of animals is that they can move of themselves. This suggested to them, especially to Aristotle, that this was a mode of movementcommon to everything that moves.

5. Heavenly bodies In contrast to “animal movement” (which was assumed to be part of all movement of objects on earth), the heavenly bodies moved very regularly. Therefore, assuming that movement requires some purpose, it was very natural for them to assume that the regular movement of the heavenly bodies was due to the will of some divine being(s) (Gods) who was responsible for this geometric simplicity and orderly movement. This is certainly a simplification, but it is probable that such beliefs formed part of what we have termed as their imaginative background, and influenced their physical picture of the world.

6. AristotleBorn: 384 BC in Stagirus, Macedonia, Greece Died: 322 BC in Chalcis, Euboea, Greece • Aristotle, more than any other thinker, determined the orientation and the content of Western intellectual history. He was the author of a philosophical and scientific system that through the centuries became the support and vehicle for both medieval Christian and Islamic scholastic thought until the end of the 17th century, Western culture was Aristotelian. And, even after the intellectual revolutions of centuries to follow, Aristotelian concepts and ideas remained embedded in Western thinking. • Encyclopaedia Britannica • Find out more at: http://www-history.mcs.st-andrews.ac.uk/history/Mathematicians/Aristotle.html

7. Aristotle • The anecdotes related of him reveal him as a kindly, affectionate character, and they show barely any trace of the self-importance that some scholars think they can detect in his works. His will, which has been preserved, exhibits the same kindly traits; he makes references to his happy family life and takes solicitous care of his children, as well as his servants. • Encyclopaedia Britannica • He was a bit of a dandy, wearing rings on his fingers and cutting his hair fashionably short. He suffered from poor digestion, and is said to have been spindle-shanked. He was a good speaker, lucid in his lectures, persuasive in conversation; and he had a mordant wit. His enemies, who were numerous, made him out to be arrogant and overbearing. ... As a man he was, I suspect, admirable rather than amiable. • J Barnes, Aristotle (Oxford, 1982) This is a detail from the fresco The School of Athens by Raphael

8. PHUSIS = physics The word for physics, in Aristotle, is the science of what the Greeks called “phusis”, which translates as “nature”. But the meaning is very different than what we mean now. Phusis was related to “growth” and has a teleological meaning – some sort of a purpose. That is, it is the sake, or the end, for which a thing exists. This nature is in the form, rather than in the matter. It involves a potential becoming, and things have some sort of a potential nature, which is an internal principle for existence. I eat; therefore, I walk, hunt, kill, .... Obvious I open & close that is my “natural” potential

9. PHUSIS = physics This view probably comes from observations in biology, and considering the imaginative surroundings of the people of this time, it is not surprising that such views would develop. This explains the attention given whether something is natural or unnatural. Even their word for “natural” incorporates this idea of fulfillment of some purpose (such as the purpose of a seed to become a plant). This conception of “nature” which embodies purpose, creates great problems when we try to develop what we now call physics. For instance “motion “ is supposed to be the fulfilling of what potentially exists. This creates problems right away, as we will see Really simple. Its obvious!

10. Why do things move? If something moves from A to B, why does it then move from B to A? An animal can move an object from A to B because it has a purpose and this movement can even be imagined to fulfill the “nature” of the animal, or maybe of the object. The motion of an animal was fulfilling the “nature” of the animal, and the motion was directed by its will. The motion of a lifeless object is explained with an extension of this concept. He suggests that the motion of inanimate objects can be explained by postulating that things tend to seek their “natural” place. What do we mean the natural place? It has to do with the order of things in the universe: water flows downhill because water want to go down. Stones move downward more strongly than water (because stones sink in water). Air goes up naturally (fire also). He had to make composite structures to explain things like wood, which have apparently both earth and air in it. But, as we will see, in the movement of lifeless objects, it only creates problems to imagine a purpose, or end, that belongs to the object which can be used to describe its motion (or even to describe the static place where it is). According to Aristotle, there is also violent motion, which is motion opposed to natural motion (such as throwing a stone into the air. The violent motion brings it up, and the natural motion brings it down. Nowadays (since Newton) we realize that gravity brings the stone down, and this is what Aristotle (and essentially many afterward) would call natural, although they had no real conception of gravity as we will explain it.

11. Aristotle’s laws of motion: 1.The speed of falling is proportional to the weight of the object. Heavier things fall faster. 2.The speed by which an object falls depends inversely on the density of the medium it is falling through. Also he asserted that during violent motion the speed of the object is directly proportional to the applied force. We will see below the fallacy of this. Because of his two laws of motion Aristotle rejects the “void”, because a void would be incompatible with his notion of how things move. According to Aristotle, something moves faster the less resistance it feels (for instance air resistance or the resistance of water as a medium). Therefore, it would move infinitely fast in a vacuum, and this is ridiculous. Note that the thought of trying to carry out some sort of experiment, by comparing quantitatively the movement in different media and then extrapolating, never occurred to him (or to the Greeks in general). But we have to give him credit, he was the first person who thought quantitatively about movement.

12. Aristotle’s thought experiment: heavy things fall faster Simple really! Just a little thinking will make things clear. For 2000 years! - heh-heh - Big things fall fast Small things fall slowly. Aristotle said so. Have they ever thought to do an experiment?

13. Aristotle’s thought experiment about falling in a medium I tell you, there can be no void or things go infinitely fast. Ridiculous!! We don’t need experiments to see that! Water Air Void!!

14. A real experiment Hey Aristotle! Did you see the experiment? Hey! Come’re! Hey - LOOK! Why is that? Water Air Void!!

15. Aristotle asserted that during violent motion the speed of the object is directly proportional to the applied force. Constant Velocity Constant Force Is this right? No! We will see below the fallacy of this (2000 years later!)

16. Aristotle’s view was a geocentric theory of the universe (we’re at the center!). There are two domains to the universe – the astronomical (heavens) and the sublunar (the earth). The astronomical realm is made of ether (this stuff is not on the earth), and the earth of earth, fire, water and air. The “natural” movement of objects in the heavens was assumed to take place on spheres. This was partly because the heavenly objects were supposed to be divine, and spheres are perfectly symmetrical shapes, corresponding to what a God would want. In this way, circular motion was said to be “natural” for heavenly bodies. In actuality, the circular motion is a continual change in the direction of motion that requires a force towards the center of the circle (gravity). Interestingly Aristotle thought that the heat from the sun was produced by some sort of friction. This was compatible with the idea that a void could not exist. The celestual motion was governed by different rules than the earthly motions.

17. Lets see how this compares to our modern classical view: Aristotle said that things move at a constant velocity because there is some action (force) that is keeping the constant motion (we will not get into the details here). This is incompatible with the first law of Newton, that any body will continue in a straight line with a constant velocity, if left to itself. We will get to this later, but of course these early ideas are in accord with what one often sees everyday, especially at that time. According to Newton’ssecond law, if a force is applied to an object, this effects a change in the velocity (acceleration). You can see the big difference in the two views. The view of Aristotle (that a force is required to keep a constant velocity) involves not only the wrong “objective” physical description (which we can attribute to the inability to understand the effect of friction) but also involves a deeper teleological view which ascribes a purposeful intention for all happenings, including the movement of rocks and other objects. Aristotles’ physics is a very different view than we have now. But we can se how easy it was to accept such a view, considering their everyday environment.

18. It is important to remember a very big difference between Plato and Aristotle: Plato thought that we could arrive at the truth by mathematical reasoning alone. Aristotle believed that empirical observations were necessary to understand what went on in the natural world (sometimes). We should say that Aristotle was much more productive, and his ideas much more useful, in biology. This was Aristotle’s great (positive) contribution to science. It was from biology that both Plato and Aristotle saw the evidence for a design in nature, which was controlled by “final causes”. Each species was to fit in the right place in the grand scheme of things. ! only ?. Then, !