A CRITIQUE OF ARISTOTLES TREATISE ON MOTION CONTAINED IN BOOK III OF HIS WORKS ON “PHYSICS”

A CRITIQUE OF ARISTOTLES TREATISE ON MOTION CONTAINED IN HIS

 “PHYSICS”

INTRODUCTION.

Unraveling the principles governing the universe has been a big puzzle for the rational man since time immemorial. In a bid to explain the workings of the universe, philosophers have propounded theories and laws based on their observation, and the power of the particular thinker to make inferences. One of such philosophers is Aristotle, who lived between 384 and 322 BC, and is said to have written about a hundred and fifty philosophical treatises, thirty of which are still extant, touching on areas ranging from biology, to physics, to morals to aesthetics to politics. 

One of Aristotle’s principal works “physics” (Gk. Phusikes meaning lectures on nature) establishes the general rules governing all natural objects, animate and inanimate, terrestrial as well as celestial. Book three of this work discusses motion of objects, in relation to the physical world.

ARISTOTLE’S CLAIMS ON MOTION.

Aristotle begins by saying that the key to understanding nature is to understand change and motion. He defines motion as the actuality of what exists potentially, in so far as it exists potentially.[1] Motion is a kind of change that is continuous since it acts on infinite objects, which are subject to place, time and space.  Thus, motion takes place within a certain place and space at a certain time.

After carefully observing the constituents of different objects in nature, Aristotle concludes that the terrestrial realm is composed of four elements which interact together: Fire, Earth, Water, and Air. Each of these elements has a natural realm. Earth’s realm is beneath our feet; water is between air, and earth; Air is around us; Fire is above air, but below the celestial realm. The celestial realm on the other hand is composed of only one substance that he calls “Aether”. He describe aether as a weightless, incorruptible, unchanging and perfect substance, that gives the celestial bodies their smooth texture and circular shape. He goes on to say that motion occurs when these natural objects strive to reach their realm in order to gain stability. This is the reason why an iron rod falls to the ground since its basic component is earth, smoke rises to the sky, since its basic component is fire, and the planetary bodies move in circle, since their basic component is aether.

He also teaches that heavier objects fall faster than lighter objects, concluding that the speed of an object in motion is directly proportional to the weight of the object, and inversely proportional to the medium through which it actualizes the motion. From this, he opines that there is no vacuum or void in nature, lest all objects will fall at infinite speed (and at the same speed) since the density of the vacuum will be zero.

He continues by saying that for a mover to move an object, there must be a kind of force applied on the object, and that the mover must be in contact with the moved throughout the period of the motion[2]. He therefore asserts that the speed of an object is directly proportional to the force applied, and motion continues as long as the mover is in contact with the moved. Also, that the object will stop instantly when force is no longer applied to the moving object, i.e. whenever the mover looses contact with the moved, the moved halts abruptly, or ceases to move. On the distance covered by an object in motion, he gives an analogy of moving an object from Athens to Thebes and back[3]. From this, it is understood that the amount of energy needed to move an object from one location A to another location B is the same as that needed to move that same body from location B to location A irrespective of external interferences.

CRITICISMS AND COUNTER ARGUMENTS.

Over the centuries, Aristotle’s works have come under great criticism from both philosophers and scientists. It is pertinent to state that the time in which Aristotle lived is different from the time in which we are living today. In fact, if Aristotle were allowed to step foot on earth in this millennia, he will be so shocked at the transformation of things, that he will have to write retractions like St Augustine did. Therefore we must be careful not to compare Aristotle’s physics with what we know today as “physics”, because Aristotle’s work is purely philosophical, stemming from observation of natural phenomenon, and conclusions based on that. On the contrary, modern physics is a positive science based on empirical verification that presupposes philosophy. We shall make a critique of Aristotle’s position on motion of natural objects from the discovery, claims and criticisms of other.

Aristotle’s definition of motion as the fulfillment of what exists potentially is faulted by Crescas, in that if the actuality is in already in the object potentially, there is no need for an external force to cause its motion. Thus if body has the potentiality (power) of rolling down a cliff, or a glass of cold water has the potentiality of becoming hot[4], why then should an external force be applied to bring this motion to actuality. Creacas helps clarify Aristotle’s definition of potentiality by saying that:

“…The term potentiality is to be understood as referring only to the potentiality of receiving motion, and not a potentiality of causing motion….”[5]

Newton’s law of universal gravitation counters Aristotle’s principle of stability of objects in motion, by stating that every object within the earth’s magnetic field is pulled towards the earth by a force originating from the earth’s core, except those that are less dense than air. This explains why solid and liquid (earth and water) objects fall to the ground, while gaseous objects (air and fire) rise up.

The great physicist Galileo Galilei (1564-1642 AD) debunks many of Aristotle’s theories, with his discoveries. According to him, Aristotle’s “Aether” doesn’t exist. Galileo’s discovery of the Telescope in 1608 helped him observe the heavenly bodies (stars and other planet), which he saw to be elliptical, and possess rough surface, in contrast to Aristotle’s perfect round and smooth bodies. He also observes that the planets revolved not because of their makeup, but because of a force of attraction from the sun, thus, confirming the hypothesis of heliocentricity earlier propounded by Nicholas Copernicus in 1616, as opposed to the ancient theory of egocentricity.(Hodgkin, 2005)

Galileo’s theory of inertia and Newton’s laws of motion counter Aristotle’s theory of force, mover and moved. According to these scientist, a body will continue to move after a force has been applied on it until it is halted be another force.  It asserts that there is no need for the mover’s continuous contact with the moved, after the force has been applied. It also brings to our consciousness the presence of other forces in nature. For Aristotle, when an arrow is propelled from a string, force is transferred from the string to the air around the base of the arrow, and so there is still contact with the mover. Accordingly, the arrow moves at a steady speed and drops instantly (at angle 90) when the mover (air as is the present case) withdraws itself. Although, he is right to assert that the medium within which motion takes place affect the speed of the motion, however, the final action of the seems rather absurd. According to Galileo and Newton, the force applied by the string accelerates the arrow (or any object) steadily, until it has reached its peak speed, before it begins to decelerate steadily. The rate of acceleration and deceleration depends on resistant forces within the medium. In the air, resistant forces come as air current or humidity; on land as frictional force; and in liquid, viscous force. On the area of differing fall of fall of objects, Galileo in his law of universal acceleration or free-fall, observed that two objects of different sizes when released from a cliff at the same time reached the bottom of the hill at the same time. Thus all objects irrespective of their size will fall at the same speed in as much as no external force is applied on them, disclaiming Aristotle’s principles that objects that differ fall at different speed.(Peterson, 2001)

In Wolfson’s book on, Crescas critique of Aristotle, it shows that from to Crescas’ view, Aristotle contradicts himself when he says that there must be contact between the mover and the moved, meaning that as the moved is in motion, the mover moves along with it. This is not in line with Aristotle’s belief in an ultimate unmoved mover,

For if every transition from potentiality to actuality is motion, then the transition of a motive agent from the state of a potential motive agent to that of an actual motive agent is motion. Every motivity then will be motion. As every motion requires a motive agent, every motivity will also require a motive agent. But this is contradictory to Aristotle’s view as to the existence of a prime immovable mover.[6]

How can Aristotle explain the attractive force that a magnet exerts on a metallic object when brought close to it? Even without having contact with it, it pulls the object to itself, and can also repel another magnet with the same polarity without actual contact with it.

It seem that during Aristotle’s youthful days he never made a trip to a mountain or highland, as to observe that one expends more energy climbing than descending a steep slope. This disparity in net energy consumed is attributed to two forces: gravity and friction. Suppose Thebes is a city on a hill and Athens below, when moving an object from Thebes to Athens, the force of gravity supports the movers force against the frictional force of the ground. However, when moving the same object from Athens to Thebes, the mover has these forces to contend with, thereby expending more energy, even though the distance remains unchanged. Or how can we explain the fact that on a snowy field, skaters can travel long distances on ice, which they cannot possibly do on a rough road or in the desert? Other than that the frictional force on ice is close to zero, as compared to that on a tarred road.

CONCLUSION.

It seems that Aristotle arrived at his conclusions by simply observing nature, without subjecting them to empirical scrutiny. He was a great philosopher, whose theories on nature, even though for more than millennia were regarded as authentic (before the advent of empirical science), have many times being refuted. The relevance of his works in the fields of science and philosophy cannot be overemphasized, being a platform upon which many discoveries have been built. He and other ancient thinkers have succeeded in handing down to generations after them the key to unraveling the mysteries of nature. Their works are sparks that lit up the torch of limitless scientific discovery, which culminated in the Enlightenment era, and continues today. Aristotle’s thinking still remains a pillar to modern scientific and philosophic scholarship.

BIBLIOGRAPHY.

1.       Aristotle, Physics. Transl. by W. D Ross. Clarendon, London: Oxford University Press, 1930.

2.       Harry Austryn Wolfson. Crescas Critique of Aristotle. Philadelphia, PA, U. S. A. The Jewish Publication Society Press, 1929.

3.       Luke Hodgkin. A History of mathematics from Mesopotamia to Modernity, New York  Oxford University Press, Inc., 2005.

4.       Mark. A. Peterson. Galileo’s Discovery of Scaling Laws, South Hadley. MA: Mount Holyoke, 2001.

5.       Vincent Edward Smith. The General Science of Nature, U. S. A. The Bruce Publishing Company, 1958.

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[1] Aristotle, Physics Bk. III, 201a, 10

[2] Aristotle, Physics Bk. III, 202a, 1

[3] Aristotle, Physics, Bk. III, 202b, 10

[4] Aristotle, Physics, Bk. III, 201a, 21

[5] Harry Austryn Wolfson. Crescas Critique of Aristotle. (Philadelphia, PA, U. S. A. The Jewish Publication  Society Press, 1929), p. 78

[6] Harry Austryn Wolfson,  Crescas Critique of Aristotle (Philadelphia, PA, USA The Jewish Publication Society Press, 1929), p. 75