Physics (Aristotle)

Physics (Aristotle)

"Physics" (or "Physica", or "Physicae Auscultationes" meaning "lessons") is a key text in the philosophy of Aristotle. It inaugurates the current Andronichean order, the long series of Aristotle's physical, cosmological and biological works, and is preliminary to them. This collection of treatises or lessons deals with theoretical, methodological, and philosophical concerns, rather than physical theories or contents of particular investigations. It sets the basis for the scientist to study the world subject to change. Change, or movement, or motion ("kinesis") is one of the chief topics of the work. The ancient Greek title of these treatises - unicode|τὰ φυσικά - meant "the [writings] on nature" or "natural philosophy".


The Physics is composed of 8 books.

Book I

Book I discusses the scientist's approach to nature and the world of changing things. Topics include: remarks on method, a discussion of how some ancestors viewed nature, and the basic elements of change. Change elements include: a property ("privation"), which is overcome by its opposite ("form"), with both of them belonging to a subject ("substrate") which is not altered in the change.

Book II

Book II introduces the term "nature" (Gr. "physis") as "the ability of setting itself in motion". Thus, those entities are natural which are capable of starting to move, e.g. growing, acquiring qualities, displacing themselves, and finally being born and dying. (Artificial entities are not born, nor can they grow or feed themselves). Here is also where Aristotle presents his theory of the four causes and the particular importance of the "final" cause. The purpose ("telos"), in nature, is stressed and contrasted with the way in which nature doesn't usually work, chance (and luck). Something happens by chance when all the stages, which would usually lead to it, "coincidentally" sum without being purposefully chosen, and produce a result similar to the teleologically caused one. This applies in human decisions as well as in nature.

Books III and IV

Books III and IV are lacking in interest and probably formed a textual whole, defining the preconditions of motion. Book III begins with a very controversial definition of change involving the metaphysical notions of potentiality and actuality. Change is the passage from being something "potentially" and becoming it "actually", and this is the structure every natural phenomenon can be reduced to. The rest of the book is a treatment of infinity, a property which no physical magnitude can have, and which (both by addition and by division) only exists "upon reflection".

Book IV discusses the preconditions of motion: place ("topos"), the various ways a thing can "be in" another (bodies can occupy place without us having to accept the existence of void), and time ("khronos"). Time is a constant attribute of movements and, Aristotle thinks, does not exist on its own but is relative to things. The relationship among time, motion, and the human soul is not univocally settled. Time is defined as "the number of movement in respect of "before" and "after", so it cannot exist without that; but it is also said that it is the soul, capable of measuring the movement, which makes there be time.

Books V and VI

Books V and VI deal with "how" motion works. Book V classifies four species of movement, depending on where the opposites are located. Movement categories include quantity (e.g. a change in dimensions, from great to small), quality (as for colors: from pale to dark), place (local movements generally go from up downwards and vice versa), or, more controversially, substance. In fact, substances do not have opposites, so it is inappropriate to say that "something" properly becomes, from not-man, man: generation and corruption are not "kinesis" in the full sense.

Book VI discusses how a changing thing can reach the opposite state, if it has to pass through infinite intermediate stages. It investigates by rational and logical arguments the notions of "continuity" and "division", establishing that change -and time, and place, consequently- are not divisible into indivisible parts; they are not mathematically discrete but continuous, infinitely divisible. This implies, in Aristotle's view, that there can be no first stage of change: there is no definite moment when the motion begins. This discussion, together with that of speed and the different behaviour of the four different species of motion, eventually helps Aristotle contrast Zeno on his claim that the existence of motion is absurd, by replying to his paradoxes.

Book VII

Book VII briefly deals with the relationship of the moved to his mover, which Aristotle describes in substantial divergence with Plato's theory of the soul as capable of setting itself in motion ("Laws" book X, "Phaedrus", "Phaedo"). Everything which moves is moved by another. He then tries to correlate the species of motion and their speeds, with the local change (locomotion, "phorà") as the most fundamental to which the others can be reduced.

Book VII has also come to us in an alternative version, not included in the Bekker edition.


Book VIII (which occupies almost a fourth of the entire "Physics", and probably constituted originally an independent course of lessons) discusses two main topics, though with a wide deployment of arguments: the time limits of the universe, and the existence of a Prime Mover — eternal, indivisible, without parts and without magnitude. Isn't the universe eternal, has it had a beginning, will it ever end? Aristotle's response, as a Greek, could hardly be affirmative, never having been told of a "creatio ex nihilo" (for the first appearance of this concept in philosophy, see St. Augustine); but he also has philosophical reasons for denying that motion didn't exist all along, on the grounds of the theory presented in the earlier books of the "Physics". Eternity of motion is also confirmed by the existence of a substance which is different from all the others in lacking matter; being pure form, it is also in an eternal actuality, not being imperfect in any respect; hence needing not to move. This is demonstrated by describing the celestial bodies thus: the first things to be moved must undergo an infinite, single and continuous movement, that is, circular. This is not caused by any contact but (I integrate the view contained in the Metaphysics, bk. XII) by love and aspiration.


"Die Aristotelische Physik", W. Wieland, 1962, 2nd revised edition 1970.

External links

* [ "Physics", trans. by R. P. Hardie and R. K. Gaye ]
* [ "Aristotle's definition of motion" and the complex 20th century debate] (since W. D. Ross). A good overview article by Joe Sachs
* [ Michael Rowan-Robinson argues that Aristotle was the first real physicist in the West.]

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