Formally, mathematics adopts an absolute theory of s.p.a.ce and time, _i.e._ it a.s.sumes that, besides the things which are in s.p.a.ce and time, there are also ent.i.ties, called "points" and "instants," which are occupied by things. This view, however, though advocated by Newton, has long been regarded by mathematicians as merely a convenient fiction.

There is, so far as I can see, no conceivable evidence either for or against it. It is logically possible, and it is consistent with the facts. But the facts are also consistent with the denial of spatial and temporal ent.i.ties over and above things with spatial and temporal relations. Hence, in accordance with Occam's razor, we shall do well to abstain from either a.s.suming or denying points and instants. This means, so far as practical working out is concerned, that we adopt the relational theory; for in practice the refusal to a.s.sume points and instants has the same effect as the denial of them. But in strict theory the two are quite different, since the denial introduces an element of unverifiable dogma which is wholly absent when we merely refrain from the a.s.sertion. Thus, although we shall derive points and instants from things, we shall leave the bare possibility open that they may also have an independent existence as simple ent.i.ties.

We come now to the question whether the things in s.p.a.ce and time are to be conceived as composed of elements without extension or duration, _i.e._ of elements which only occupy a point and an instant. Physics, formally, a.s.sumes in its differential equations that things consist of elements which occupy only a point at each instant, but persist throughout time. For reasons explained in Lecture IV., the persistence of things through time is to be regarded as the formal result of a logical construction, not as necessarily implying any actual persistence. The same motives, in fact, which lead to the division of things into point-particles, ought presumably to lead to their division into instant-particles, so that the ultimate _formal_ const.i.tuent of the matter in physics will be a point-instant-particle. But such objects, as well as the particles of physics, are not data. The same economy of hypothesis, which dictates the practical adoption of a relative rather than an absolute s.p.a.ce and time, also dictates the practical adoption of material elements which have a finite extension and duration. Since, as we saw in Lecture IV., points and instants can be constructed as logical functions of such elements, the mathematical account of motion, in which a particle pa.s.ses continuously through a continuous series of points, can be interpreted in a form which a.s.sumes only elements which agree with our actual data in having a finite extension and duration. Thus, so far as the use of points and instants is concerned, the mathematical account of motion can be freed from the charge of employing fictions.

(d) But we must now face the question: Is there, in actual empirical fact, any sufficient reason to believe the world of sense continuous?

The answer here must, I think, be in the negative. We may say that the hypothesis of continuity is perfectly consistent with the facts and with logic, and that it is technically simpler than any other tenable hypothesis. But since our powers of discrimination among very similar sensible objects are not infinitely precise, it is quite impossible to decide between different theories which only differ in regard to what is below the margin of discrimination. If, for example, a coloured surface which we see consists of a finite number of very small surfaces, and if a motion which we see consists, like a cinematograph, of a large finite number of successive positions, there will be nothing empirically discoverable to show that objects of sense are not continuous. In what is called _experienced_ continuity, such as is said to be given in sense, there is a large negative element: absence of perception of difference occurs in cases which are _thought_ to give perception of absence of difference. When, for example, we cannot distinguish a colour A from a colour B, nor a colour B from a colour C, but can distinguish A from C, the indistinguishability is a purely negative fact, namely, that we do not _perceive_ a difference. Even in regard to immediate data, this is no reason for denying that there is a difference. Thus, if we see a coloured surface whose colour changes gradually, its sensible appearance if the change is continuous will be indistinguishable from what it would be if the change were by small finite jumps. If this is true, as it seems to be, it follows that there can never be any empirical evidence to demonstrate that the sensible world is continuous, and not a collection of a very large finite number of elements of which each differs from its neighbour in a finite though very small degree.

The continuity of s.p.a.ce and time, the infinite number of different shades in the spectrum, and so on, are all in the nature of unverifiable hypotheses--perfectly possible logically, perfectly consistent with the known facts, and simpler technically than any other tenable hypotheses, but not the sole hypotheses which are logically and empirically adequate.

If a relational theory of instants is constructed, in which an "instant"

is defined as a group of events simultaneous with each other and not all simultaneous with any event outside the group, then if our resulting series of instants is to be compact, it must be possible, if _x_ wholly precedes _y_, to find an event _z_, simultaneous with part of _x_, which wholly precedes some event which wholly precedes _y_. Now this requires that the number of events concerned should be infinite in any finite period of time. If this is to be the case in the world of one man's sense-data, and if each sense-datum is to have not less than a certain finite temporal extension, it will be necessary to a.s.sume that we always have an infinite number of sense-data simultaneous with any given sense-datum. Applying similar considerations to s.p.a.ce, and a.s.suming that sense-data are to have not less than a certain spatial extension, it will be necessary to suppose that an infinite number of sense-data overlap spatially with any given sense-datum. This hypothesis is possible, if we suppose a single sense-datum, _e.g._ in sight, to be a finite surface, enclosing other surfaces which are also single sense-data. But there are difficulties in such a hypothesis, and I do not know whether these difficulties could be successfully met. If they cannot, we must do one of two things: either declare that the world of one man's sense-data is not continuous, or else refuse to admit that there is any lower limit to the duration and extension of a single sense-datum. I do not know what is the right course to adopt as regards these alternatives. The logical a.n.a.lysis we have been considering provides the apparatus for dealing with the various hypotheses, and the empirical decision between them is a problem for the psychologist.

(3) We have now to consider the _logical_ answer to the alleged difficulties of the mathematical theory of motion, or rather to the positive theory which is urged on the other side. The view urged explicitly by Bergson, and implied in the doctrines of many philosophers, is, that a motion is something indivisible, not validly a.n.a.lysable into a series of states. This is part of a much more general doctrine, which holds that a.n.a.lysis always falsifies, because the parts of a complex whole are different, as combined in that whole, from what they would otherwise be. It is very difficult to state this doctrine in any form which has a precise meaning. Often arguments are used which have no bearing whatever upon the question. It is urged, for example, that when a man becomes a father, his nature is altered by the new relation in which he finds himself, so that he is not strictly identical with the man who was previously not a father. This may be true, but it is a causal psychological fact, not a logical fact. The doctrine would require that a man who is a father cannot be strictly identical with a man who is a son, because he is modified in one way by the relation of fatherhood and in another by that of sonship. In fact, we may give a precise statement of the doctrine we are combating in the form: _There can never be two facts concerning the same thing._ A fact concerning a thing always is or involves a relation to one or more ent.i.ties; thus two facts concerning the same thing would involve two relations of the same thing. But the doctrine in question holds that a thing is so modified by its relations that it cannot be the same in one relation as in another.

Hence, if this doctrine is true, there can never be more than one fact concerning any one thing. I do not think the philosophers in question have realised that this is the precise statement of the view they advocate, because in this form the view is so contrary to plain truth that its falsehood is evident as soon as it is stated. The discussion of this question, however, involves so many logical subtleties, and is so beset with difficulties, that I shall not pursue it further at present.

When once the above general doctrine is rejected, it is obvious that, where there is change, there must be a succession of states. There cannot be change--and motion is only a particular case of change--unless there is something different at one time from what there is at some other time. Change, therefore, must involve relations and complexity, and must demand a.n.a.lysis. So long as our a.n.a.lysis has only gone as far as other smaller changes, it is not complete; if it is to be complete, it must end with terms that are not changes, but are related by a relation of earlier and later. In the case of changes which appear continuous, such as motions, it seems to be impossible to find anything other than change so long as we deal with finite periods of time, however short. We are thus driven back, by the logical necessities of the case, to the conception of instants without duration, or at any rate without any duration which even the most delicate instruments can reveal. This conception, though it can be made to seem difficult, is really easier than any other that the facts allow. It is a kind of logical framework into which any tenable theory must fit--not necessarily itself the statement of the crude facts, but a form in which statements which are true of the crude facts can be made by a suitable interpretation. The direct consideration of the crude facts of the physical world has been undertaken in earlier lectures; in the present lecture, we have only been concerned to show that nothing in the crude facts is inconsistent with the mathematical doctrine of continuity, or demands a continuity of a radically different kind from that of mathematical motion.

LECTURE VI

THE PROBLEM OF INFINITY CONSIDERED HISTORICALLY

It will be remembered that, when we enumerated the grounds upon which the reality of the sensible world has been questioned, one of those mentioned was the supposed impossibility of infinity and continuity. In view of our earlier discussion of physics, it would seem that no _conclusive_ empirical evidence exists in favour of infinity or continuity in objects of sense or in matter. Nevertheless, the explanation which a.s.sumes infinity and continuity remains incomparably easier and more natural, from a scientific point of view, than any other, and since Georg Cantor has shown that the supposed contradictions are illusory, there is no longer any reason to struggle after a finitist explanation of the world.

The supposed difficulties of continuity all have their source in the fact that a continuous series must have an infinite number of terms, and are in fact difficulties concerning infinity. Hence, in freeing the infinite from contradiction, we are at the same time showing the logical possibility of continuity as a.s.sumed in science.

The kind of way in which infinity has been used to discredit the world of sense may be ill.u.s.trated by Kant's first two antinomies. In the first, the thesis states: "The world has a beginning in time, and as regards s.p.a.ce is enclosed within limits"; the ant.i.thesis states: "The world has no beginning and no limits in s.p.a.ce, but is infinite in respect of both time and s.p.a.ce." Kant professes to prove both these propositions, whereas, if what we have said on modern logic has any truth, it must be impossible to prove either. In order, however, to rescue the world of sense, it is enough to destroy the proof of _one_ of the two. For our present purpose, it is the proof that the world is _finite_ that interests us. Kant's argument as regards s.p.a.ce here rests upon his argument as regards time. We need therefore only examine the argument as regards time. What he says is as follows:

"For let us a.s.sume that the world has no beginning as regards time, so that up to every given instant an eternity has elapsed, and therefore an infinite series of successive states of the things in the world has pa.s.sed by. But the infinity of a series consists just in this, that it can never be completed by successive synthesis. Therefore an infinite past world-series is impossible, and accordingly a beginning of the world is a necessary condition of its existence; which was the first thing to be proved."

Many different criticisms might be pa.s.sed on this argument, but we will content ourselves with a bare minimum. To begin with, it is a mistake to define the infinity of a series as "impossibility of completion by successive synthesis." The notion of infinity, as we shall see in the next lecture, is primarily a property of _cla.s.ses_, and only derivatively applicable to series; cla.s.ses which are infinite are given all at once by the defining property of their members, so that there is no question of "completion" or of "successive synthesis." And the word "synthesis," by suggesting the mental activity of synthesising, introduces, more or less surrept.i.tiously, that reference to mind by which all Kant's philosophy was infected. In the second place, when Kant says that an infinite series can "never" be completed by successive synthesis, all that he has even conceivably a right to say is that it cannot be completed _in a finite time_. Thus what he really proves is, at most, that if the world had no beginning, it must have already existed for an infinite time. This, however, is a very poor conclusion, by no means suitable for his purposes. And with this result we might, if we chose, take leave of the first antinomy.

It is worth while, however, to consider how Kant came to make such an elementary blunder. What happened in his imagination was obviously something like this: Starting from the present and going backwards in time, we have, if the world had no beginning, an infinite series of events. As we see from the word "synthesis," he imagined a mind trying to grasp these successively, _in the reverse order_ to that in which they had occurred, _i.e._ going from the present backwards. _This_ series is obviously one which has no end. But the series of events up to the present has an end, since it ends with the present. Owing to the inveterate subjectivism of his mental habits, he failed to notice that he had reversed the sense of the series by subst.i.tuting backward synthesis for forward happening, and thus he supposed that it was necessary to identify the mental series, which had no end, with the physical series, which had an end but no beginning. It was this mistake, I think, which, operating unconsciously, led him to attribute validity to a singularly flimsy piece of fallacious reasoning.

The second antinomy ill.u.s.trates the dependence of the problem of continuity upon that of infinity. The thesis states: "Every complex substance in the world consists of simple parts, and there exists everywhere nothing but the simple or what is composed of it." The ant.i.thesis states: "No complex thing in the world consists of simple parts, and everywhere in it there exists nothing simple." Here, as before, the proofs of both thesis and ant.i.thesis are open to criticism, but for the purpose of vindicating physics and the world of sense it is enough to find a fallacy in _one_ of the proofs. We will choose for this purpose the proof of the ant.i.thesis, which begins as follows:

"a.s.sume that a complex thing (as substance) consists of simple parts.

Since all external relation, and therefore all composition out of substances, is only possible in s.p.a.ce, the s.p.a.ce occupied by a complex thing must consist of as many parts as the thing consists of. Now s.p.a.ce does not consist of simple parts, but of s.p.a.ces."

The rest of his argument need not concern us, for the nerve of the proof lies in the one statement: "s.p.a.ce does not consist of simple parts, but of s.p.a.ces." This is like Bergson's objection to "the absurd proposition that motion is made up of immobilities." Kant does not tell us why he holds that a s.p.a.ce must consist of s.p.a.ces rather than of simple parts.

Geometry regards s.p.a.ce as made up of points, which are simple; and although, as we have seen, this view is not scientifically or logically _necessary_, it remains _prima facie_ possible, and its mere possibility is enough to vitiate Kant's argument. For, if his proof of the thesis of the antinomy were valid, and if the ant.i.thesis could only be avoided by a.s.suming points, then the antinomy itself would afford a conclusive reason in favour of points. Why, then, did Kant think it impossible that s.p.a.ce should be composed of points?

I think two considerations probably influenced him. In the first place, the essential thing about s.p.a.ce is spatial order, and mere points, by themselves, will not account for spatial order. It is obvious that his argument a.s.sumes absolute s.p.a.ce; but it is spatial _relations_ that are alone important, and they cannot be reduced to points. This ground for his view depends, therefore, upon his ignorance of the logical theory of order and his oscillations between absolute and relative s.p.a.ce. But there is also another ground for his opinion, which is more relevant to our present topic. This is the ground derived from infinite divisibility. A s.p.a.ce may be halved, and then halved again, and so on _ad infinitum_, and at every stage of the process the parts are still s.p.a.ces, not points. In order to reach points by such a method, it would be necessary to come to the end of an unending process, which is impossible. But just as an infinite cla.s.s can be given all at once by its defining concept, though it cannot be reached by successive enumeration, so an infinite set of points can be given all at once as making up a line or area or volume, though they can never be reached by the process of successive division. Thus the infinite divisibility of s.p.a.ce gives no ground for denying that s.p.a.ce is composed of points. Kant does not give his grounds for this denial, and we can therefore only conjecture what they were. But the above two grounds, which we have seen to be fallacious, seem sufficient to account for his opinion, and we may therefore conclude that the ant.i.thesis of the second antinomy is unproved.

The above ill.u.s.tration of Kant's antinomies has only been introduced in order to show the relevance of the problem of infinity to the problem of the reality of objects of sense. In the remainder of the present lecture, I wish to state and explain the problem of infinity, to show how it arose, and to show the irrelevance of all the solutions proposed by philosophers. In the following lecture, I shall try to explain the true solution, which has been discovered by the mathematicians, but nevertheless belongs essentially to philosophy. The solution is definitive, in the sense that it entirely satisfies and convinces all who study it carefully. For over two thousand years the human intellect was baffled by the problem; its many failures and its ultimate success make this problem peculiarly apt for the ill.u.s.tration of method.

The problem appears to have first arisen in some such way as the following.[22] Pythagoras and his followers, who were interested, like Descartes, in the application of number to geometry, adopted in that science more arithmetical methods than those with which Euclid has made us familiar. They, or their contemporaries the atomists, believed, apparently, that s.p.a.ce is composed of indivisible points, while time is composed of indivisible instants.[23] This belief would not, by itself, have raised the difficulties which they encountered, but it was presumably accompanied by another belief, that the number of points in any finite area or of instants in any finite period must be finite. I do not suppose that this latter belief was a conscious one, because probably no other possibility had occurred to them. But the belief nevertheless operated, and very soon brought them into conflict with facts which they themselves discovered. Before explaining how this occurred, however, it is necessary to say one word in explanation of the phrase "finite number." The _exact_ explanation is a matter for our next lecture; for the present, it must suffice to say that I mean 0 and 1 and 2 and 3 and so on, for ever--in other words, any number that can be obtained by successively adding ones. This includes all the numbers that can be expressed by means of our ordinary numerals, and since such numbers can be made greater and greater, without ever reaching an unsurpa.s.sable maximum, it is easy to suppose that there are no other numbers. But this supposition, natural as it is, is mistaken.

[22] In what concerns the early Greek philosophers, my knowledge is largely derived from Burnet's valuable work, _Early Greek Philosophy_ (2nd ed., London, 1908). I have also been greatly a.s.sisted by Mr D. S.

Robertson of Trinity College, who has supplied the deficiencies of my knowledge of Greek, and brought important references to my notice.

[23] _Cf._ Aristotle, _Metaphysics_, M. 6, 1080b, 18 _sqq._, and 1083b, 8 _sqq._

Whether the Pythagoreans themselves believed s.p.a.ce and time to be composed of indivisible points and instants is a debatable question.[24]

It would seem that the distinction between s.p.a.ce and matter had not yet been clearly made, and that therefore, when an atomistic view is expressed, it is difficult to decide whether particles of matter or points of s.p.a.ce are intended. There is an interesting pa.s.sage[25] in Aristotle's _Physics_,[26] where he says:

"The Pythagoreans all maintained the existence of the void, and said that it enters into the heaven itself from the boundless breath, inasmuch as the heaven breathes in the void also; and the void differentiates natures, as if it were a sort of separation of consecutives, and as if it were their differentiation; and that this also is what is first in numbers, for it is the void which differentiates them."

[24] There is some reason to think that the Pythagoreans distinguished between discrete and continuous quant.i.ty. G. J. Allman, in his _Greek Geometry from Thales to Euclid_, says (p. 23): "The Pythagoreans made a fourfold division of mathematical science, attributing one of its parts to the how many, t? p?s??, and the other to the how much, t?

p??????; and they a.s.signed to each of these parts a twofold division.

For they said that discrete quant.i.ty, or the _how many_, either subsists by itself or must be considered with relation to some other; but that continued quant.i.ty, or the _how much_, is either stable or in motion. Hence they affirmed that arithmetic contemplates that discrete quant.i.ty which subsists by itself, but music that which is related to another; and that geometry considers continued quant.i.ty so far as it is immovable; but astronomy (t?? sfa??????) contemplates continued quant.i.ty so far as it is of a self-motive nature. (Proclus, ed.

Friedlein, p. 35. As to the distinction between t? p??????, continuous, and t? p?s??, discrete quant.i.ty, see Iambl., _in Nicomachi Geraseni Arithmeticam introductionem_, ed. Tennulius, p. 148.)" _Cf._ p. 48.

[25] Referred to by Burnet, _op. cit._, p. 120.

[26] iv., 6. 213b, 22; H. Ritter and L. Preller, _Historia Philosophiae Graecae_, 8th ed., Gotha, 1898, p. 75 (this work will be referred to in future as "R. P.").

This seems to imply that they regarded matter as consisting of atoms with empty s.p.a.ce in between. But if so, they must have thought s.p.a.ce could be studied by only paying attention to the atoms, for otherwise it would be hard to account for their arithmetical methods in geometry, or for their statement that "things are numbers."

The difficulty which beset the Pythagoreans in their attempts to apply numbers arose through their discovery of incommensurables, and this, in turn, arose as follows. Pythagoras, as we all learnt in youth, discovered the proposition that the sum of the squares on the sides of a right-angled triangle is equal to the square on the hypotenuse. It is said that he sacrificed an ox when he discovered this theorem; if so, the ox was the first martyr to science. But the theorem, though it has remained his chief claim to immortality, was soon found to have a consequence fatal to his whole philosophy. Consider the case of a right-angled triangle whose two sides are equal, such a triangle as is formed by two sides of a square and a diagonal. Here, in virtue of the theorem, the square on the diagonal is double of the square on either of the sides. But Pythagoras or his early followers easily proved that the square of one whole number cannot be double of the square of another.[27] Thus the length of the side and the length of the diagonal are incommensurable; that is to say, however small a unit of length you take, if it is contained an exact number of times in the side, it is not contained any exact number of times in the diagonal, and _vice versa_.

[27] The Pythagorean proof is roughly as follows. If possible, let the ratio of the diagonal to the side of a square be _m_/_n_, where _m_ and _n_ are whole numbers having no common factor. Then we must have _m_2 = 2_n_2. Now the square of an odd number is odd, but _m_2, being equal to 2_n_2, is even. Hence _m_ must be even. But the square of an even number divides by 4, therefore _n_2, which is half of _m_2, must be even. Therefore _n_ must be even. But, since _m_ is even, and _m_ and _n_ have no common factor, _n_ must be odd. Thus _n_ must be both odd and even, which is impossible; and therefore the diagonal and the side cannot have a rational ratio.

Now this fact might have been a.s.similated by some philosophies without any great difficulty, but to the philosophy of Pythagoras it was absolutely fatal. Pythagoras held that number is the const.i.tutive essence of all things, yet no two numbers could express the ratio of the side of a square to the diagonal. It would seem probable that we may expand his difficulty, without departing from his thought, by a.s.suming that he regarded the length of a line as determined by the number of atoms contained in it--a line two inches long would contain twice as many atoms as a line one inch long, and so on. But if this were the truth, then there must be a definite numerical ratio between any two finite lengths, because it was supposed that the number of atoms in each, however large, must be finite. Here there was an insoluble contradiction. The Pythagoreans, it is said, resolved to keep the existence of incommensurables a profound secret, revealed only to a few of the supreme heads of the sect; and one of their number, Hippasos of Metapontion, is even said to have been shipwrecked at sea for impiously disclosing the terrible discovery to their enemies. It must be remembered that Pythagoras was the founder of a new religion as well as the teacher of a new science: if the science came to be doubted, the disciples might fall into sin, and perhaps even eat beans, which according to Pythagoras is as bad as eating parents' bones.

The problem first raised by the discovery of incommensurables proved, as time went on, to be one of the most severe and at the same time most far-reaching problems that have confronted the human intellect in its endeavour to understand the world. It showed at once that numerical measurement of lengths, if it was to be made accurate, must require an arithmetic more advanced and more difficult than any that the ancients possessed. They therefore set to work to reconstruct geometry on a basis which did not a.s.sume the universal possibility of numerical measurement--a reconstruction which, as may be seen in Euclid, they effected with extraordinary skill and with great logical ac.u.men. The moderns, under the influence of Cartesian geometry, have rea.s.serted the universal possibility of numerical measurement, extending arithmetic, partly for that purpose, so as to include what are called "irrational"

numbers, which give the ratios of incommensurable lengths. But although irrational numbers have long been used without a qualm, it is only in quite recent years that logically satisfactory definitions of them have been given. With these definitions, the first and most obvious form of the difficulty which confronted the Pythagoreans has been solved; but other forms of the difficulty remain to be considered, and it is these that introduce us to the problem of infinity in its pure form.

We saw that, accepting the view that a length is composed of points, the existence of incommensurables proves that every finite length must contain an infinite number of points. In other words, if we were to take away points one by one, we should never have taken away all the points, however long we continued the process. The number of points, therefore, cannot be _counted_, for counting is a process which enumerates things one by one. The property of being unable to be counted is characteristic of infinite collections, and is a source of many of their paradoxical qualities. So paradoxical are these qualities that until our own day they were thought to const.i.tute logical contradictions. A long line of philosophers, from Zeno[28] to M. Bergson, have based much of their metaphysics upon the supposed impossibility of infinite collections.

Broadly speaking, the difficulties were stated by Zeno, and nothing material was added until we reach Bolzano's _Paradoxien des Unendlichen_, a little work written in 1847-8, and published posthumously in 1851. Intervening attempts to deal with the problem are futile and negligible. The definitive solution of the difficulties is due, not to Bolzano, but to Georg Cantor, whose work on this subject first appeared in 1882.

[28] In regard to Zeno and the Pythagoreans, I have derived much valuable information and criticism from Mr P. E. B. Jourdain.

In order to understand Zeno, and to realise how little modern orthodox metaphysics has added to the achievements of the Greeks, we must consider for a moment his master Parmenides, in whose interest the paradoxes were invented.[29] Parmenides expounded his views in a poem divided into two parts, called "the way of truth" and "the way of opinion"--like Mr Bradley's "Appearance" and "Reality," except that Parmenides tells us first about reality and then about appearance. "The way of opinion," in his philosophy, is, broadly speaking, Pythagoreanism; it begins with a warning: "Here I shall close my trustworthy speech and thought about the truth. Henceforward learn the opinions of mortals, giving ear to the deceptive ordering of my words."

What has gone before has been revealed by a G.o.ddess, who tells him what really _is_. Reality, she says, is uncreated, indestructible, unchanging, indivisible; it is "immovable in the bonds of mighty chains, without beginning and without end; since coming into being and pa.s.sing away have been driven afar, and true belief has cast them away." The fundamental principle of his inquiry is stated in a sentence which would not be out of place in Hegel:[30] "Thou canst not know what is not--that is impossible--nor utter it; for it is the same thing that can be thought and that can be." And again: "It needs must be that what can be thought and spoken of is; for it is possible for it to be, and it is not possible for what is nothing to be." The impossibility of change follows from this principle; for what is past can be spoken of, and therefore, by the principle, still is.

[29] So Plato makes Zeno say in the _Parmenides_, apropos of his philosophy as a whole; and all internal and external evidence supports this view.

[30] "With Parmenides," Hegel says, "philosophising proper began."

_Werke_ (edition of 1840), vol. xiii. p. 274.

The great conception of a reality behind the pa.s.sing illusions of sense, a reality one, indivisible, and unchanging, was thus introduced into Western philosophy by Parmenides, not, it would seem, for mystical or religious reasons, but on the basis of a logical argument as to the impossibility of not-being. All the great metaphysical systems--notably those of Plato, Spinoza, and Hegel--are the outcome of this fundamental idea. It is difficult to disentangle the truth and the error in this view. The contention that time is unreal and that the world of sense is illusory must, I think, be regarded as based upon fallacious reasoning.

Nevertheless, there is some sense--easier to feel than to state--in which time is an unimportant and superficial characteristic of reality.

Past and future must be acknowledged to be as real as the present, and a certain emanc.i.p.ation from slavery to time is essential to philosophic thought. The importance of time is rather practical than theoretical, rather in relation to our desires than in relation to truth. A truer image of the world, I think, is obtained by picturing things as entering into the stream of time from an eternal world outside, than from a view which regards time as the devouring tyrant of all that is. Both in thought and in feeling, to realise the unimportance of time is the gate of wisdom. But unimportance is not unreality; and therefore what we shall have to say about Zeno's arguments in support of Parmenides must be mainly critical.

The relation of Zeno to Parmenides is explained by Plato[31] in the dialogue in which Socrates, as a young man, learns logical ac.u.men and philosophic disinterestedness from their dialectic. I quote from Jowett's translation: