A Source Book in Greek Science

Author: Lucretius  | Date: 1921

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The Kinetics of Atoms in the System of Epicurus

Lucretius, On the Nature of Things II. 80–164, 216–332.1 Translation of Cyril Bailey (Oxford, 1921)

If you think that the first-beginnings of things can stay still, and by staying still beget new movements in things, you stray very far away from true reasoning. For since they wander through the void, it must needs be that all the first-beginnings of things move on either by their own weight2 or sometimes by the blow of another. For when quickly, again and again, they have met and clashed together, it comes to pass that they leap asunder at once this way and that; for indeed it is not strange, since they are most hard with solid heavy bodies, and nothing bars them from behind. And the more you perceive all the bodies of matter tossing about, bring it to mind that there is no lowest point in the whole universe, nor have the first-bodies any place where they may come to rest, since I have shown in many words, and it has been proved by true reasoning, that space spreads out without bound or limit, immeasurable towards every quarter everywhere. And since that is certain, no rest, we may be sure, is allowed to the first-bodies moving through the deep void, but rather plied with unceasing, diverse motion, some when they have dashed together leap back at great space apart, others too are thrust but a short way from the blow. And all those which are driven together in more close-packed union and leap back but a little space apart, entangled by their own close-locking shapes, these make the strong roots of rock and the brute bulk of iron and all other things of their kind. Of the rest which wander through the great void, a few leap far apart, and recoil afar with great spaces between; these supply for us thin air and the bright light of the sun. Many, moreover, wander on through the great void, which have been cast back from the unions of things, nor have they anywhere else availed to be taken into them and link their movements. And of this truth, as I am telling it, a likeness and image is ever passing presently before our eyes. For look closely, whenever rays are let in and pour the sun’s light through the dark places in houses: for you will see many tiny bodies mingle in many ways all through the empty space right in the light of the rays, and as though in some everlasting strife wage war and battle, struggling troop against troop, nor ever crying a halt, harried with constant meetings and partings; so that you may guess from this what it means that the first-beginnings of things are for ever tossing in the great void. So far as may be, a little thing can give a picture of great things and afford traces of a concept. And for this reason it is the more right for you to give heed to these bodies, which you see jostling in the sun’s rays, because such jostlings hint that there are movements of matter too beneath them, secret and unseen. For you will see many particles there stirred by unseen blows change their course and turn back, driven backwards on their path, now this way, now that, in every direction everywhere. You may know that this shifting movement comes to them all from the first-beginnings. For first the first-beginnings of things move of themselves; then those bodies which are formed of a tiny union, and are, as it were, nearest to the powers of the first-beginnings, are smitten and stirred by their unseen blows, and they in their turn, rouse up bodies a little larger. And so the movement passes upwards from the first-beginnings, and little by little comes forth to our senses, so that those bodies move too, which we can descry in the sun’s light; yet it is not clearly seen by what blows they do it.1

Next, what speed of movement is given to the first-bodies of matter, you may learn, Memmius, in a few words from this. First, when dawn strews the land with new light, and the diverse birds flitting through the distant woods across the soft air fill the place with their clear cries, we see that it is plain and evident for all to behold how suddenly the sun is wont at such a time to rise and clothe all things, bathing them in his light. And yet that heat which the sun sends out, and that calm light of his, is not passing through empty space; therefore, it is constrained to go more slowly, while it dashes asunder, as it were, the waves of air. Nor again do the several particles of heat move on one by one, but entangled one with another, and joined in a mass; therefore they are at once dragged back each by the other, and impeded from without, so that they are constrained to go more slowly. But the first-beginnings, which are of solid singleness, when they pass through the empty void, and nothing checks them without, and they themselves, single wholes with all their parts, are borne, as they press on, towards the one spot which they first began to seek, must needs, we may be sure, surpass in speed of motion, and be carried far more quickly than the light of the sun, and rush through many times the distance of space in the same time in which the flashing light of the sun crowds the sky.

Herein I would fain that you should learn this, too, that when first-bodies are being carried downwards straight through the void by their own weight, at times quite undetermined, and at undetermined spots they push a little from their path:2 yet only just so much as you could call a change of trend. But if they were not used to swerve, all things would fall downwards through the deep void like drops of rain, nor could collision come to be, nor a blow brought to pass for the first-beginnings: so nature would never have brought aught to being.

But if perchance any one believes that heavier bodies, because they are carried more quickly straight through the void, can fall from above on the lighter, and so bring about the blows which can give creative motions, he wanders far away from true reason. For all things that fall through the water and thin air, these things must needs quicken their fall in proportion to their weights, just because the body of water and the thin nature of air cannot check each thing equally, but give place more quickly when overcome by heavier bodies. But, on the other hand, the empty void cannot on any side, at any time, support anything, but rather, as its own nature desires, it continues to give place; wherefore all things must needs be borne on through the calm void, moving at equal rate with unequal weights.1 The heavier will not then ever be able to fall on the lighter from above, nor of themselves bring about the blows, which make diverse the movements, by which nature carries things on. Wherefore, again and again, it must needs be that the first-bodies swerve a little; yet not more than the very least, lest we seem to be imagining a sideways movement, and the truth refute it. For this we see plain and evident, that bodies, as far as in them lies, cannot travel sideways, since they fall headlong from above, as far as you can descry. But that nothing at all swerves from the straight direction of its path, what sense is there which can descry?

Once again, if every motion is always linked on, and the new always arises from the old in order determined, nor by swerving do the first-beginnings make a certain start of movement to break through the decrees of fate, so that cause may not follow cause from infinite time; whence comes this free will for living things all over the earth, whence, I ask, is it wrested from fate, this will whereby we move forward, where pleasure leads each one of us, and swerve likewise in our motions neither at determined times nor in a determined direction of place, but just where our mind has carried us? For without doubt it is his own will which gives to each one a start for this movement, and from the will the motions pass flooding through the limbs. Do you not see too how, when the barriers are flung open, yet for an instant of time the eager might of the horses cannot burst out so suddenly as their mind itself desires? For the whole store of matter throughout the whole body must be roused to movement, that then aroused through every limb it may strain and follow the eager longing of the mind; so that you see a start of movement is brought to pass from the heart, and comes forth first of all from the will of the mind, and then afterwards is spread through all the body and limbs. Nor is it the same as when we move forward impelled by a blow from the strong might and strong constraint of another. For then it is clear to see that all the matter of the body moves and is hurried on against our will, until the will has reined it back throughout the limbs. Do you not then now see that, albeit a force outside pushes many men and constrains them often to go forward against their will and to be hurried away headlong, yet there is something in our breast, which can fight against it and withstand it? And at its bidding too the store of matter is constrained now and then to turn throughout the limbs and members, and, when pushed forward, is reined back and comes to rest again. Wherefore in the seeds too you must needs allow likewise that there is another cause of motion besides blows and weights, whence comes this power born in us, since we see that nothing can come to pass from nothing. For weight prevents all things coming to pass by blows, as by some force without. But that the very mind feels not some necessity within in doing all things, and is not constrained like a conquered thing to bear and suffer, this is brought about by the tiny swerve of the first-beginnings in no determined direction of place and at no determined time.

Nor was the store of matter ever more closely packed nor again set at larger distances apart. For neither does anything come to increase it nor pass away from it. Wherefore the bodies of the first-beginnings in the ages past moved with the same motion as now, and hereafter will be borne on for ever in the same way; such things as have been wont to come to being will be brought to birth under the same law, will exist and grow and be strong and lusty, inasmuch as is granted to each by the ordinances of nature. Nor can any force change the sum of things; for neither is there anything outside, into which any kind of matter may escape from the universe, nor whence new forces can arise and burst into the universe and change the whole nature of things and alter its motions.1

Herein we need not wonder why it is that, when all the first-beginnings of things are in motion, yet the whole seems to stand wholly at rest, except when anything starts moving with its entire body. For all the nature of the first-bodies lies far away from our senses, below their purview; wherefore, since you cannot reach to look upon them, they must needs steal away their motions from you too; above all, since such things as we can look upon, yet often hide their motions, when withdrawn from us on some distant spot. For often the fleecy flocks cropping the glad pasture on a hill creep on whither each is called and tempted by the grass bejewelled with fresh dew, and the lambs fed full gambol and butt playfully; yet all this seems blurred to us from afar, and to lie like a white mass on a green hill. Moreover, when mighty legions fill the spaces of the plains with their chargings, awaking a mimic warfare, a sheen rises there to heaven and all the earth around gleams with bronze, and beneath a noise is roused by the mighty mass of men as they march, and the hills smitten by their shouts turn back the cries to the stars of the firmament, and the cavalry wheel round and suddenly shake the middle of the plains with their forceful onset, as they scour across them. And yet there is a certain spot on the high hills, whence all seems to be at rest and to lie like a glimmering mass upon the plains.

2 Herein lies an important difference between Democritean and Epicurean atomism. In the former system weight is neither a primary quality of the atom nor a cause of motion. Weight first enters into the system when a world is in the process of being formed, and then only as a measure of the degree of resistance to the attendant whirl, wherein central and outer parts are differentiated. Before that stage, the motion of the atom is not due to weight but is a specific characteristic of atomicity and there is no differentiation of "up" and "down." The system of Epicurus differs, as our text indicates. [Edd.]

1 Just as free atoms move with an incomprehensibly great velocity, so the atoms within compounds retain this velocity as they are jostled about by the other atoms within the compound. The extent of the trajectories depends on the closeness of the union. The compound, of course, whose component atoms are thus jostled in all directions, cannot itself be said to move with atomic velocity. Instead, as the atoms unite to form larger and larger bodies, the motion of the latter is retarded more and more until finally a body is produced large enough to be perceived and with motion slow enough to be perceived. Such is the dust particle in the sunbeam—a favorite example of the ancient atomists. Note throughout the emphasis on analogies. [Edd.]

2 Having assumed that all free atoms fall in the same direction (downward) in the void, and at equal speed, Epicurus, in order to account for the initial impacts of atom upon atom, further assumes that at unpredictable times and places an atom may swerve ever so slightly from the perpendicular. Again, this swerve without external cause releases the universe from the inexorable chain of causality, introduces an indeterminism not present in the Democritean system, and accounts for the free will manifested by living beings. Analogies have sometimes been drawn between certain features of modern physical theory and the abandonment of strict causality in the system of Epicurus. [Edd.]

1 In this paragraph we have the doctrine of conservation of matter and momentum in its simplest form. [Edd.]

1 This is the fundamental proposition about freely falling bodies in our physics. It was to avoid the consequences of its application to freely falling atoms that Epicurus introduced the "swerve" (see also p. 206). [Edd.]


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Chicago: Lucretius, "The Kinetics of Atoms in the System of Epicurus," A Source Book in Greek Science, trans. Cyril Bailey in A Source Book in Greek Science, ed. Morris R. Cohen and I. E. Drabkin (Cambridge: Harvard University Press, 1948), 212–217. Original Sources, accessed May 20, 2024, http://originalsources.com/Document.aspx?DocID=G2384S1SIV1NVWF.

MLA: Lucretius. "The Kinetics of Atoms in the System of Epicurus." A Source Book in Greek Science, translted by Cyril Bailey, Vol. II., in A Source Book in Greek Science, edited by Morris R. Cohen and I. E. Drabkin, Cambridge, Harvard University Press, 1948, pp. 212–217. Original Sources. 20 May. 2024. http://originalsources.com/Document.aspx?DocID=G2384S1SIV1NVWF.

Harvard: Lucretius, 'The Kinetics of Atoms in the System of Epicurus' in A Source Book in Greek Science, trans. . cited in 1948, A Source Book in Greek Science, ed. , Harvard University Press, Cambridge, pp.212–217. Original Sources, retrieved 20 May 2024, from http://originalsources.com/Document.aspx?DocID=G2384S1SIV1NVWF.