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Aristotle’s cosmology and physics

Page history last edited by Marek 15 years, 7 months ago



     Aristotle is reputed to be one of the most influential of the ancient Greek philosophers, but the scope of Aristotle's work was not merely limited to philosophy. It also extended into a number of different areas such as biology, where he performed careful observations and classifications of animals, and physics, where he studied the motion of objects. In addition to this, Aristotle developed substantial systems for understanding and explaining natural phenomena which have been studied, critiqued, and transcribed to the present day.


     Two factors that made Aristotle's natural philosophy extremely attractive for centuries to come were his empiricism (common sense approach based on observation) and his interlocking view of the cosmos.  Most notably his concept of a geocentric universe and his system of elements shaped the works of intellectuals in western civilization through the Middle Ages and up to the Reniassance. The foundations laid by Aristotle's system of reasoning are imprinted into the modern scientific method.


Aristotle's Life


     Aristotle was born in 384 BC(E). He was sent to Plato's Academy and spent 20 years there. Foremost, Aristotle was a student of biology and his biological investigations largely shaped his worldview. He travelled Asia Minor (present day Turkey) and classified over 500 organisms in his lifetime. He also became the tutor of a young Alexander the Great in Macedonia.


Note: We do not actually have any full dialogues or texts by Aristotle. We only have copies of what would be the equivalent of his lecture notes, which is one of the reasons Aristotle's works are rather difficult to read. 


Aristotle's Philisophical Underpinnings 


     In ontology, Aristotle was a materialist, as opposed to an idealist like Plato. To him, what exists is what can be experienced or perceived through the senses; true existence is in what is here and now. This viewpoint was highly opposed to Plato's belief that the senses corrupt visions of the ideal concepts in our minds; that true reality only exists outside of the perceivable (the theory of Forms). For Aristotle, attributes of “horseness” do not exist independently as a Form in another realm. Instead of replicating imperfectly in individual horses, they exist in every horse. The properties that define an object as part of a group are inseparable from it and are called essential properties. The properties that pertain only to a specific individual object from the group are called accidental properties.


     The existence of properties outside of an object, which Plato’s Forms allowed, was deemed impossible by Aristotle. There is no goodness he argued, there only exist good things. A property requires a subject. Aristotle called the collection of properties of an object its form. The subject of these properties is matter. Matter brings no properties of its own to the object. Of course, the two cannot be separated from the object, and indeed both are a real part of its existence.


     Aristotle's epistemology, though heavily influenced by his teacher, Plato, also shows some significant differences. Aristotle was an empiricist, not a rationalist. He believed knowledge is obtained through careful observation and understanding of causes (thus his works are of a teleological nature). Through the study of individuals, one gathers sense experiences and transfers them to memory. If one has enough insight, from the experiences can be extracted universal features of things. These universal features allow the construction of definitions, which can then be used to make judgments by deduction. Aristotle devoted much attention to proper forms of deductive reasoning in his works. He can be said to be the founder of logic. To construct the definitions though, one needs induction.


Causation and syllogistic argument


     Aristotle was, as has been said, very interested in the study of causation. In order to have a cause, according to Aristotle, one must be able to place an argument in a categorical syllogism, such as:


All men are mortal.

Socrates is a man

therefore, Socrates is mortal.


This form, now classical to Aristotelian logic, creates a series of premises, one major and one minor, that lead to a conclusion by form and form alone (given that the premises are true). Socrates is a member of the class of men, and all men are members of the class of mortals, thus Socrates must be a member of that class as well. The fact that Socrates is a man is the cause for him being mortal.





     Until Aristotle, the way philosophers dealt with the problem of change was to deem it illusory, as it is only a result of change in composition, or to remove it from true reality, as did Plato. Aristotle affirmed the existence of change. Change in his philosophy became real. This of course subjected him to much criticism.


     First of all, matter does not change in Aristotle’s view. Only the form is replaced by another one. This requires explaining where the new form comes from, for it cannot be generated from nothing. To explain this, Aristotle introduced a third category of being. Now there is nonbeing, potential being, and actual being. The addition of potentiality removes the need for nonbeing to be evoked in the discussion of change. Change simply becomes the realization of a potentiality which already existed in the object. For example, an acorn has the potentiality to become an oak tree. The growth of the acorn, its change, is simply a realization of this potentiality.


     Another question arises though, why would change occur? What is the reason for the potentiality to be realized? Aristotle’s answer is that the driving force behind change is the object’s natural tendency. An object behaves the way it does because it is in its nature to do so. Of course these natural tendencies can meet with resistance or be interfered with. One should also note that artificial objects do not have natural sources of change, and the nature of a complex object is not a sum of its constituents’ natures.


     Aristotle believed that there were four types of causes which helped make the clear distinction between potentiality and actuality (an acorn in actuality is just a nut, but it is potentially a tree). The fundamental causes consist of: material cause (what a substance is made out of), formal cause (what a substance started out being, and what it became), efficient cause (who, what made the substance), final cause (purpose of the substance).  Together these causes are a formula for describing the natural world.




     Aristotle also dealt with cosmology. He divided the universe into two realms, each with their own unique properties: the terrestrial realm, which included everything under the moon, and the celestial realm, which was everything from the moon to the fixed stars.


Terrestrial Realm


     Aristotle adopted the four elementals (earth, water, air, and fire) proposed by Empedocles as the components of the terrrestrial realm. These elements, he claimed, were combinations of the paradigms: hot/cold and wet/dry. By displaying how these basic elements could shift by increasing or decreasing heat and wetness, Aristotle displayed a mechanism by which he could explain change. These elements were also sorted by their weight. He classified air and fire as being "naturally light" and sought their natural state away from the center of the earth.  Earth and water, however, were considered to be "naturally heavy" and tended towards the center.

     The natural motion of the heavy elements toward a common center was one argument Aristotle gave for the spherical shape of the Earth. Other arguments include:

- The circular shadow cast by the Earth during a lunar eclipse is circular

- Moving in the north-south direction changes the position of the stars

- Ships seam to sink under the horizon


 Celestial Realm


     Outside of the terrestrial realm was the moon, Mercury and Venus, the Sun, Mars, Jupiter, and Saturn, and finally the stars which were fixed and without motion. The celestial realm had different properties from the sublunar realm. Aristotle theorized it was made of a fifth element, or quintessence, which had a natural circular motion. He believed that the Earth was at the center of the universe and that nothing existed beyond the fixed stars on the outside spere of the cosmos except the Unmoved Mover.  What Aristotle saw as "nothing" was not the vacuum we think of today, but literally the absence of anything except the Unmoved Mover.


     The planets, and the Sun revolved around the Earth. Everything that moves, according to Aristotle, must be driven by a force. So there must have been something unmoved, which caused the eternal motion of the bodies in the celestial realm. Aristotle creates an Unmoved Mover for this purpose– a deity representing the highest good, wholly actualized, and separated from the spheres it moves. This Prime Mover then cannot be the efficient cause of the motion (lack of contact) and must therefore be its final cause. Every celestial body has an Unmoved Mover as its object of desire. The purpose of the circular motion is to imitate the perfection of the Unmoved Mover (Lindberg).


Aristotle's other pursuits




     In studying motion, Aristotle made a fundamental distinction between natural motion and violent motion. Natural motion, he said, was motion brought about by the innate physical nature of the substance (e.g., heavy objects' natural tendency to fall).  Violent motion occurred when force acted on an object to move it contrary to its natural motion (as when an object is thrown).  In all cases, Aristotle advocated the central principle that no motion could occur without a mover, be it an internal or external agent. For example, when a projectile is thrust into the air, the force is imparted to the surrounding air, as well as to the object itself. The air then takes over as the mover throughout the projectile’s motion. The motion continues until the air is able to support it.


     The two motions: violent and natural, were discussed seperately and different conclusions were arrived at. Through the natural motion of the elements, Aristotle introduced absolute lightness and absolute heaviness. The concepts of density and relative weight existed and were known to Aristotle, but he thought they played no role in determining a body's natural motion. Considering an object in free-fall, he claimed that:

- Velocity is directly proportional to the weight of the body (an object of twice the weight falls with twice the speed)

- The time of motion is inversely proportional to the weight (an object of half the weight falls twice as long)

Aristotle did specify that all natural motions are accelerated motions, but then proceeded to talk only about velocity as though the motion was of uniform speed.


     Considering violent motion, Aristotle discusses time and distance, without mentioning velocity. Among his observations regarding violent motion are:

- The time to move a certain distance is proportional to the motive force

- The time to travel a certain distance is inversely proportional to the resistive power of the medium

- When motive and resistant forces are equal or the former is less than the latter, the body does not move

- Equal force will move half the original weight, twice the distance.


     When criticizing these observations, it is important to remember that Aristotle only had a clear concept of distance and time. Velocity or speed was not employed in the same manner as today. The same thing may be said about force and weight.




     Aristotle worked extensively in the field of biology as well.  He developed a classification system (taxonomy) for animals based on the whether they were blooded or non-blooded, and their method of birthing young (viviporous, oviporous, etc.), and also created a hierarchical system for all living things.  Using this system, he recognized that whales and dolphins were not fish.  Using meticulous dissection techniques, Aristotle also closely studied chicken embryos at various stages of development to better understand the process.  His careful studies and observations of various marine and other animals were influential in the zoological field for many centuries.  Due to these observations Aristotle favored the model of epigenesis (the idea that tissues and organs appear sequentially during development) rather than preformation (the idea that all tissues and organs are present, but tiny, from fertilization and simply grow during development).



Primary Sources:




Key Terms and Definitions:


empiricism: common sense approach based on observation


material cause: what a substance is made out of


formal cause: what a substance started out being and what it turned into


efficient cause: who/what made the substance


final cause: purpose of the substance


natural motion: an object motion w/o being acted upon by an outside force 


violent motion: an object's motion when it has been acted upon by an outside force


quintessence: Aristotle's 5th element; what the celestial realm was made out of


sublunar realm: everything under the moon (sublunar: latin for 'under the moon')


celestial realm: everything from the moon to the stars and beyond


terrestial realm: realm of the elements


epigenesis: the idea that tissues and organs appear sequentially during developement


preformation: the idea that all tissues and organs are present, but tiny, from fertilization and simply grow during developement


essential properties: properties that define the object (for example: a car has wheels, door(s), an engine, windows/glass etc.)


accidental properties: properties that happen to characterize an object ( for example: a car's color, make etc.)



Relevant Links:

Internet Encyclopedia of Philosophy-Aristotle:   http://www.iep.utm.edu/a/aristotl.htm

Internet Classics Archive-Works by Aristotle:  http://classics.mit.edu/Browse/browse-Aristotle.html


Comments (21)

Douglas Elliott said

at 12:15 am on Sep 5, 2008

Sorry this is still a work in progress; I"ve been experiencing major computer issues. Please stand by for more updates tonight!

Grant Berry said

at 8:49 am on Sep 5, 2008

I tried to thros in a little about his epistemology and some talk on his logic, as it is very important. If need be, later I'll develop a little bit more on Aristotelian logics. Check the grammar though. There are a couple of rough spots, I think.

lindsey said

at 11:16 am on Sep 5, 2008

I added a few things about Aristotle's four causes and the movement of objects. It's also kind of a rough outline and will be correcting them later.

sdn646@... said

at 3:55 pm on Sep 5, 2008

I found a good picture of how Aristotle viewed the celestial realm. You have to click on the "See full-size image" at the top to see a less blurry picture though.

jgm829@... said

at 8:17 pm on Sep 5, 2008

I added a little on Aristotle's life and explained the difference in form and matter.

Larry_Crump said

at 3:35 pm on Sep 6, 2008

Hola all. I went ahead and edited this article for spelling and grammar is soon to come.

Alyson Collins said

at 5:06 pm on Sep 6, 2008

I corrected some grammar, but was at a loss how to piece together a lot of it. Any grammatarian please have at it. I also kinda left it looking like it was chopped up into tiny pieces (seperated paragraphs and labeled/made new sections), and didn't fill it in much. But I figure ya'll probaly will. And I'll be back later after my brain stops hurting! :)

Alyson Collins said

at 5:10 pm on Sep 6, 2008

I also noticed that someone accidentally deleted the picture, the primary sources, definitions etc section. If someone wants to put them back on, that'd be great!

Alyson Collins said

at 6:05 pm on Sep 7, 2008

I noticed that the info under Nat Philosophy just repeated basically what was said in the element theory section and other parts. But never fear I didn't change anything but grammar, sentence variation, and typos this time because I thought someone might be still working on it. It'll need fixed though.

Garrett McCormack said

at 6:27 pm on Sep 7, 2008

I added a little about Aristotle's biology

liz mastroianni said

at 8:46 pm on Sep 7, 2008

i just added a couple things because it was very well written and didn't really need any adding too

Amanda Beattie said

at 1:41 pm on Sep 8, 2008

Since we don't have a definition section this week, I went through and made important terms bold. I also added some information on Aristotle's natural philosophy, and other sections.

Alyson Collins said

at 3:58 pm on Sep 8, 2008

I added some relevant links (note on the one for Aristotle's works when you click it, it says the page can't be found, that's because it incorrectly transfers the URL, make sure its Browse/browse not just browse/). I also made a key term area and transfered all our terms in bold down there also.

Nicole Hagstrom said

at 5:27 pm on Sep 8, 2008

I made a few grammatical shifts and spelling changes and also tweaked the section on elemental theory.

Garry Polley said

at 7:16 pm on Sep 8, 2008

I created and added the diagram in the book to help explain Aristotle's explanation of the elements and how they change.

Keriann Collins said

at 8:19 pm on Sep 8, 2008

I just tried to edit things a bit. This summary is enormous, but I think that is Aristotle's fault. Overall, the content was good; the most major changes I made were to the section formerly labelled "Aristotle's Epistemology." I changed it to "Aristotle's Philosophical Underpinnings" and made some distinctions within the section between epistemology and ontology, which seemed to have gotten somewhat muddled. I quit after "Causation and Syllogistic Argument," though; I've already been at it an hour.
-Keri Collins

Grant Berry said

at 8:42 pm on Sep 8, 2008

I just have to say, great job on the diagram, Garry!

Kristy Carey said

at 12:56 am on Sep 9, 2008

I added more about biology and made overall editting/stylistic changes.

Nathan Keller said

at 2:56 am on Sep 9, 2008

I reworked the motion section some, though I think we could still do more to it. Also, I collected some of the bits on the Unmoved Mover, added a bit, and moved them. That's right. I moved the Unmoved Mover.

Marek said

at 1:10 am on Sep 14, 2008

I rearranged and relabelled things to help the flow of ideas; added a discussion about change, which was missing, and a few more of Aristotle's observations on motion; also fixed some errors throughout the summary.

Marek said

at 12:04 pm on Oct 14, 2008

Clarified Aristotle's views on motion, which might be helpful for a comparison with those of the Middle Ages.

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