Five-minute digest
Posterior Analytics and Physics
Summary
This volume joins two treatises by Aristotle that together set out what scientific knowledge is and how the natural world is to be understood. The Posterior Analytics, in two books, belongs to the logical works later grouped as the Organon. It opens with the claim that all teaching and learning by argument proceed from knowledge already possessed. We know a thing scientifically, Aristotle says, when we know the cause on which it depends and that it could not be otherwise, and such knowledge comes through demonstration: a syllogism whose premisses are true, primary, immediate, better known than and prior to the conclusion, and causes of it. Book I argues against an infinite regress of proofs and against circular demonstration, distinguishes knowing a fact from knowing the reasoned fact (the planets are near because they do not twinkle), compares sciences that borrow principles from one another, as when the physician knows that circular wounds heal slowly and the geometer knows why, and notes that loss of a sense means loss of the knowledge that depends on it. Book II treats the four questions of inquiry (whether a thing is, what it is, that a fact holds, why it holds), shows that the cause is the middle term of a demonstration, and discusses definition. Its final chapter explains how the first principles themselves are known: from repeated sense-perception arise memory and experience, and from experience the universal, which is finally grasped by intuition (nous), like a routed army rallying as one soldier after another makes a stand. The Physics, in eight books, applies this ideal to nature. Book I examines the earlier philosophers and identifies matter, form and privation as the principles of change. Book II defines nature as an inner source of motion and rest, sets out the four causes (material, formal, efficient and final), discusses chance and spontaneity, and argues that nature acts for an end, as the swallow builds its nest and plants send roots downward. Book III defines motion as the actuality of the potential as such and examines the infinite; Book IV treats place, the void and time, defined as the number of motion with respect to before and after. Books V and VI classify kinds of change and address continuity and Zeno's paradoxes. Books VII and VIII argue that motion is eternal and that every chain of movers must end in a first mover that is itself unmoved, one, eternal, and without parts or magnitude. The library text is the Oxford translation edited by W. D. Ross: G. R. G. Mure's Posterior Analytics (1928) and R. P. Hardie and R. K. Gaye's Physics (1930).
Top Lessons
To know is to know the cause
Aristotle distinguishes knowing that something is so from knowing why it must be so. The physician may know from experience that circular wounds heal slowly; the geometer knows the reason. Scientific knowledge, in his strict sense, is the second kind.
Knowledge begins in the senses
Posterior Analytics II.19 traces knowledge from perception to memory, from memory to experience, and from experience to the universal grasped by intuition. Aristotle rejects both innate knowledge in a finished form and an endless chain of proofs.
Nature acts for an end
In Physics II Aristotle argues against those who explain natural things by necessity and chance alone. Regular outcomes, such as the swallow's nest, the spider's web and the downward roots of plants, show purpose in nature even though nature does not deliberate.
Every series of movers needs a first
Physics VIII argues that motion is eternal and that the chain of things moved by others cannot be infinite, so there must be a first mover that is itself unmoved. Medieval thinkers, Jewish, Muslim and Christian, adapted the argument to reason about God.
Verified Quotes
“By demonstration I mean a syllogism productive of scientific knowledge”
“But induction is impossible for those who have not sense-perception.”
Frequently cited
“It is like a rout in battle stopped by first one man making a stand and then another, until the original formation has been restored. The soul is so constituted as to be capable of this process.”
“since except intuition nothing can be truer than scientific knowledge, it will be intuition that apprehends the primary premisses”
“Now what is to us plain and obvious at first is rather confused masses, the elements and principles of which become known to us later by analysis.”
Frequently cited
“If then it is both by nature and for an end that the swallow makes its nest and the spider its web, and plants grow leaves for the sake of the fruit and send their roots down (not up) for the sake of nourishment, it is plain that this kind of cause is operative in things which come to be and are by nature.”
“Now mistakes come to pass even in the operations of art : the grammarian makes a mistake in writing and the doctor pours out the wrong dose.”
Frequently cited
“It is plain then that nature is a cause, a cause that operates for a purpose.”
“It is clear, therefore, that the first movent is indivisible and is without parts and without magnitude.”
Medical Lens
Aristotle was a physician's son, and medicine supplies many of his examples. In the Posterior Analytics he uses medicine to show how sciences borrow principles: the physician knows that circular wounds heal more slowly, the geometer knows why, a distinction between empirical and causal knowledge that later physicians, including Galen, discussed at length. He argues that losing a sense removes the knowledge that depends on it, a point relevant to the history of thinking about sensory disability. In the Physics he illustrates the four causes with health as the end of walking about, and with reducing flesh, purging, drugs and surgical instruments as means toward health; he compares nature's purposive action to a doctor treating himself, and explains malformed births as failures of a purposive process, just as the doctor sometimes pours the wrong dose. His teleology of organs and functions became the framework of Galen's physiology and of medieval medicine. These are philosophical uses of medical examples, not clinical teaching.
- Posterior Analytics I.13 (79a)
The physician knows that circular wounds heal more slowly; the geometer knows the reason why, an example of one science taking its reasons from another.
- Posterior Analytics I.18 (81a–b)
Loss of any sense entails loss of a corresponding portion of knowledge, since universals are grasped by induction from perception.
- Physics II.3 (194b–195a)
Health as the final cause of walking; reducing flesh, purging, drugs and surgical instruments as means for the sake of health.
- Physics II.8 (199a–b)
The doctor who pours out the wrong dose and the malformations of early creatures as failures of purposive effort; nature compared to a doctor doctoring himself.