Part II: PLATO & ARISTOTLE Death of Socrates (David, 1787) - - PowerPoint PPT Presentation

Death of Socrates (David, 1787)

Part II: PLATO & ARISTOTLE

PCES: 1.21

Plato: the Quest after Ultimate Truth

Socrates (470-399 BC) Plato (428-348 BC)

Plato was heavily influenced by Socrates when young, and also by the death of

• Socrates. Socrates was condemned byan Athenian court to die (and did so by

voluntarily drinkinghemlock, refusing an offer to escape made by friends). The charge was corrupting the young (which he had done by philosophizing, in a way described in Plato’s dialogues). This has immortalized the “Socratic method”, which tries to elucidate a problem, or the truth of an idea, by dialectic (question and answer) inquiry. The writings of Plato (which we apparently have in their entirety) are very wide-ranging. We are here concerned with those parts relevant to the physical world and our knowledge of it. We will concentrate on the “Theory of Forms” and its implications. This theory is introduced on the next slide, and discussed extensively in the Course Notes. Apart from Socrates, Plato was heavily influenced in the formulation of his ideas by (i) the ideas of Heraclitus & Parmenides on change vs. constancy, and (ii) the developments in mathematics (notably by his friend Thaetetus, and by the earlier Pythagorean school). This led to an interest in geometry, including solid geometric figures – although it is not clear how much Plato himself was involved in the study of these.

PCES: 1.22 "W "Who then a ho then are lovers e lovers of

• f wisd

wisdom (philo

• m (philosophers)?

sophers)? Those who seek to d Those who seek to discern the scern the ultim ultimate na ate nature of reality." ture of reality." Plato, Plato, The Commonw The Commonwealth ealth

Plato’s Cave allegory likens our sense perceptions to the shadows of real objects cast upon the wall of a cave. The real objects, in this allegory, represent the higher “Forms” of which we have no direct perception. True knowledge is then knowleDge of the archetypal forms themselves, which are real, eternal, & unchanging. Sense perception, then, does not give us access to reality, but only to an impermanent world of perception. In modern physics, the ultimate constituents of matter resemble Platonic Forms: one deals with Fields and Probability amplitudes, as we will see. The irony is that the existence of these very abstract entities has been discovered by us, not by philosophical speculation or ratiocination, but instead by a combination of mathematical theory and experiment: ie., by a combination of the kind of mathematical deduction that Plato envisaged, together with the kind of experimentation that he apparently would have ruled to be irrelevant.

The key idea in The key idea in the “Theo the “Theory o y of F f Forms” s” was was existence of a supra- existence of a supra-sensible realm of sensible realm of “ideas” “ideas”

• r “Forms

r “Forms”, beyond the world of ”, beyond the world of

• appearances. The argument for this was
• appearances. The argument for this was

essentially one of abstraction from particular essentially one of abstraction from particular imper imperfe fect instan ct instances o ces of th things in ings in the sensible the sensible world (eg., objects that were approximately world (eg., objects that were approximately cir circular cular), to the ‘real’ ), to the ‘real’ things, the universal things, the universal ‘F ‘Forms’, like ‘Circle’, which could only be

• rms’, like ‘Circle’, which could only be

defined in the higher world of Forms. defined in the higher world of Forms. From ideas about simple Forms like “Circle’, From ideas about simple Forms like “Circle’, Plato went on to discuss higher forms, Plato went on to discuss higher forms, culminating in the high culminating in the highest of all: the “Good”. est of all: the “Good”.

PLATO: the THEORY of “FORMS” PLATO: the THEORY of “FORMS”

PCES: 1.23

Aristotle (384-322 BC) Aristotle & Alexander

Aristotle: the Real World

Aristotle was the most illustrious student of Plato; he wasa pupil in his school “the Academy” and later founded his own (the “Lyceum”). He himself was a teacher to the youngAlexander the Great- who later conquered & changed the whole of the known world as far as India, Vastly extending the influence of Greek ideas & culture (perhapsin line with Plato’s ideas on the role of education!). Theinfluence of Aristotle on later European culture was colossal. Aristotle classified and organized the whole of Greek thinking, in a way so comprehensive and detailed, & with such perception, that modern education is still designed along the lines he laid out. We only have later versions of his writings, the originals being lost (in, eg., the fire in Alexandria). For us the principal interest of Aristotle is in his denial of Plato’s supra- sensible world- he argues instead for a single physical world. The fundamental “stuff” of this, which he called “substance”. Any object in the world was described in terms of 4 ‘Causes’: Material Cause: the matter from which it is made Formal Cause: the form the matter takes Efficient Cause: the influences/agencies acting to change the object Final Cause: the purpose or goal of the object and of the changes Only one of these (the efficient cause) conforms to the modern use of the term ‘cause’. For Aristotle, it was impossible to separate any of these causes from the others – it was meaningless to talk about ‘form’ or ‘Forms” separately from the rest. The fundamental nature, including the very existence of an object, resulted from its 4 Causes.

PCES: 1.24

Aristotelian Picture of Nature

PCES: 1.25

In In Ar Aristotle’s theo istotle’s theory, the 4 , the 4 Empedo Empedoclean clean elements (earth, lements (earth, water, air water, air, , & fi & fire) ) ar are in e involved lved in th in the M e Mater terial ial Cause of an object, but th Cause of an object, but these only ese only exist in the sub-lunar realm. The exist in the sub-lunar realm. The superlunar superlunar realm was a plenum of realm was a plenum of the 5 the 5th

th

element (quintessence). The element (quintessence). The 4 sub-lunar elements are 4 sub-lunar elements are themselves made fr themselves made from various prop

• m various propor
• rtions of more fundamental pairs

tions of more fundamental pairs

• f opp
• f opposites such as hot –
• sites such as hot –

cold, or

• ld, or wet –

wet – dry (the Fig. at right shows ry (the Fig. at right shows how this was done), and also heavy – how this was done), and also heavy – light. light.

There w as There w as no v no void in id in the univ the univers erse according to Ar according to Aris istotle. totle. Nevertheles Nevertheless change w a change w as pos possible ble – – there w ere both ‘natur here w ere both ‘natural’ al’ and arti and artificial cial changes changes of

• f objects
• bjects,

including their including their motion.

• motion. Natura

Natural changes occurred under th changes occurred under the inf e influence of luence of the 4 the 4 Causes

• Causes. How ever

How ever it w as w as als also pos possible for the chan ible for the changes ges or motion to be ‘

• r motion to be ‘forced

forced’, ’, ie. ie., not conforming to the not conforming to the fundamental natu fundamental nature of re of the object. the object. For exampl For example, e, if if a a javel javelinis inis throw n, throw n, according to Ari according to Aristotle totle the the motion is motion is forced w hen forced w hen throw n, but throw n, but thereafter continues thereafter continues under the under the action of action of the s the surrounding rrounding medium, w hich pushes medium, w hich pushes it it along.

• along. Thus

Thus for Ar for Aris istotle no continuou totle no continuous motion could occur w ithout motion could occur w ithout a a force acting continuous force acting continuously ly on the object

• n the object. Alt

Although Aris hough Aristotle did not descr totle did not describe things ibe things quantitat quantitativ ively ely, he w as he w as often say

• ften saying that

ing that v = v = F / m / m w here w here v v is is the velocity the velocity, , m m the mass the mass, & , & F F the total the total force on the ob force on the object.

• ject. This

This force included force included both the applied both the applied force force and the res and the resistance from ance from the medium. the medium. Un Unfortunatel fortunately he is he is unc unclear on this lear on this – – thus thus in the dis in the discus ussion of

• n of the s

the superlunary uperlunary realm, realm, w here there is w here there is no res no resistance to ance to motion, motion, he argues he argues that there is that there is no no force s force since otherw is nce otherw ise the the

• bject w ould mov
• bject w ould move at an in

at an infinite v inite velocity locity, a , a statement incompatible w i atement incompatible w ith the equation above, th the equation above, ins instead impl tead implying that ng that v = F v = F / R / R w here w here R R is is the res the resis istance to motion tance to motion from from the medium. the medium. I It has has to be adm to be admitted that here, tted that here, as as w ell as w ell as in his in his distinction be distinction betw een natural tw een natural & forced motion, forced motion, Ar Aris istotle’ totle’s ideas ideas are a bit v are a bit vague ague and incoherent ( and incoherent (see Co ee Cours urse notes) notes). For For his Cos Cosmology

• logy, s

see e later on. later on.