Summary:

• It is the nature of atoms to fall downwards.
• Since there are worlds which exist, some atoms must swerve, otherwise none would ever have met.
• As a result of the swerves, they crash into each other.
• The speed of atoms.
• There was never a time at which processes of destruction and generation of objects composed of atoms did not exist.
• The swerve of atoms is random.



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"Thus Epicurus can say that all objects, because of their weight, fall in a particular direction in relation to himself when he stands upright on the surface of the earth, which is in its turn in the centre of the world.  Since the earth is at the centre of the world, we should notice that the words 'up' and 'down' need have no relation to positions nearer or further from the edge of the world.  Their meaning is entirely dependent on us as observers.  In a sense we are a fixed point, or at least standing on a fixed point.  That is the importance of the notion that the earth is at the centre of our world.  But what about our world in the universe as a whole ?  We have seen that the words 'up' and 'down' are conventional terms referring to directions in relation to us.   Heavy objects do in fact fall in a particular direction.  We call that downwards.  What is to prevent us applying these terms in the universe as a whole ?  Since physical objects fall in one direction, that is 'down' in our world, therefore atoms fall 'down' in our world.  And if atoms fall 'down' in our world, then there is no counter-evidence to suggest that they do anything else anywhere else.  Here Epicurus' canonic can come to the rescue of his physics.  At any rate we know that as a result of some such argumentation Epicurus concluded that free atoms have one direction, 'downwards'.


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"Now we have already observed that, if there had been a time at which all the atoms were falling perpendicularly downwards at uniform speed, worlds could never have formed since the atoms would never have met.  But worlds in fact exist and atomic speed does not vary; therefore, Epicurus held, the atoms cannot all have been constantly moving perpendicularly downwards.  Hence arises the need for a minute swerve of atoms in the atomic rain.  This swerve, as Lucretius tells us, takes place at no fixed time and at no fixed place.  Its existence is necessitated by the evidence of the senses that there is a world and that therefore all atoms cannot always have been falling perpendicularly downwards.  Obviously in some sense the rain of atoms is prior to the swerve, but that priority need not be atemporal priority.  We shall return to this later.  For the time being we can observe that, once the swerve has occurred, the swerving atoms will collide with other atoms and a third type of atomic movement will be set up.  The atoms will be jolted from their downward paths by blows from all sides.  We should notice that Epicurus rules out another way in which the down-falling atoms could meet.  He rejects the idea that the heavier atoms might catch up the lighter. For, he believes, in the void all atoms must travel at the same speed.


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"We must now consider the third type of force that affects the motion of atoms, the 'blows' which arise when the atoms collide, in the first instance because of the swerve.  At this stage we should recall that we are still dealing with the movements of individual 'free' atoms, not of atomic compounds.  It should be clear that as a result of the swerve atoms floating in the void may be subject to blows from any direction and their own direction correspondingly varied.  Of course, the weight of the atom is still also affecting its motion and will come entirely into its own again if the pressure of blows happens to be relaxed.  But in the case of very light atoms the effect of weight will be so far nullified by that of 'blows' that if a light atom is trapped between two heavy ones and they clash together, it can be driven in an 'upward' direction.


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"Despite all the blows which an atom experiences, however, its speed is not affected. In whatever direction it goes, it will be 'travelling through void 'as quickly as thought'.  Epicurus probably believed that the speed of atoms, even if deflected, is uniform for the same reasons that he believed that in the void the downward speed of two atoms of different weights is the same.  It has been argued that he made this assertion because otherwise he could not have replied to Aristotelian criticism of earlier atomists and would have had to abandon his thesis of indivisibility, but we need not assume that he found his solution to this problem difficult to swallow.  In the world of atomism there is no reason why 'free' atoms should not move at uniform speeds in the void.  What is to stop them ?, Epicurus might have said.  We can certainly afford to give him the credit of thinking along these lines.


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"We can now return to a question which we had to defer until we had outlined all the factors which contribute to the infinitely complex movements of Epicurean atoms: weight, the swerve,and the collisions of the atoms with one another.  The question is that of the origin of disorderly motion.  Was there ever a time when there was no universe ?  All the atoms were raining downwards because of their weight.  No swerve had taken place, and hence no collisions.  According to a recent critic, who finds the idea of the logical, rather than the historical priority of the perpendicular fall of the atoms to their swerve attractive, there are no Epicurean texts to guide us on this point.  'We do not know', concludes Merlan, 'whether Epicurus was aware of this problem, nor, if he was, how he solved it.  'Earlier, others have been bolder.  Both Liepmann and Krokiewicz argued that the priority of the downward fall is only logical, and Krokiewicz went on to draw the proper inference that, if the priority of the downward fall due to weight is only logical, there must always have been a series of worlds in existence.  And despite Merlan the position of Liepmann and Krokiewicz is substantially correct.  Lucretius provides the evidence.  In one passage he tells us that the processes of generation and destruction have existed from infinite time (ex infinito tempore); in another we read that the atoms have been driven both by blows and by weight from infinite time, so that every possible combination of atoms has already come into existence.  Kleve has added an a priori argument which points in the same direction.  Since the Epicureans have no doubt that time has no beginning, and since time can only be understood in relation to the existence of physical compounds of atoms, these compounds themselves must always have existed.  There seems to be no reason to suppose therefore that the cosmological processes had a beginning in time.  There have always been worlds and there always will be.  The priority of the downward fall of atoms to the swerve is a logical priority only.


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The causes of atomic motion, therefore, are weight, the swerve, and the collisions of atoms.  But, as has often been noticed, Epicurus and the Epicureans sometimes talk about chance as a cause, or at least appear to do so.  Particularly in the ethical writings we find exhortations to endure the rebuffs of chance and fortune.  Sometimes this means little more than that we must be prepared to meet unexpected, though not uncaused events.  We may similarly discount such phrases as Lucretius' fortuna gubernans ["fortune steering"] in a passage where the poet is hoping that the end of the world will not occur in his own life-time.  Fortuna here is a personification of what will happen; the Stoics talked of Fate in similar ways.  But in the letter to Menoeceus (133-4) there are more serious problems.  Epicurus seems to be saying that the causes of events are either necessity, chance, or human action.  We can postpone an enquiry into the origins of human behaviour until we treat Epicurus' psychological theories in more detail; and by 'necessity' Epicurus obviously refers to the observable and definable laws of nature, thefoedera naturai, as Lucretius calls them.  What then does he mean by chance ?  Two possibilities are immediately ruled out.  Chance is not a god, as the general public may believe.  Nor is it an unstable cause.  It is not clear whose views Epicurus is attacking here.  He may have Democritus in mind (rightly or wrongly), or he may be thinking of the 'errant' cause in Plato'sTimaeus.  Unfortunately in the letter to Menoeceus he does not develop his own view of chance in physics.  He contents himself with saying that human behaviour does not depend on chance circumstances.   It is not a matter of chance whether we behave well or ill.  Rather chance may provide us with opportunities which we can use as we wish.  Since in this whole section Epicurus is contrasting the effects of 'chance' both with those of formulable natural laws and with acts which are 'in our power' entirely, he must be thinking neither of the effects of the weight of the atoms nor of the swerve in the atoms of the soul.  What he is concerned with is the effect of the swerve in nature, that is, with those collisions of atoms which are not to be explained in terms of any mathematical formula or comprehensible law.  There are some atomic movements which can in no way be forecast.  Obviously the general behaviour of complexes of atoms can be forecast, but 'free' atoms are not susceptible to the necessity which must largely apply to atoms in groups; and even within a group there is always the limited possibility of random atomic behaviour.  As Plutarch puts it, 'an atom swerves to the very smallest extent in order that the heavy bodies, living things and chance may come into existence and that what is in our power may not perish'.  This passage is not isolated, as is sometimes suggested.  It fits into Epicurus' account of the movement of atoms.  The swerve is necessitated by the fact of the existence of worlds, but the swerve must of necessity be random.  There is no reason to distrust Plutarch's suggestion that Epicurus associated 'chance' with swerves.  His rejection of the 'Democritean' or 'Platonic' notion that chance is an 'unstable cause' need have nothing to do with this question. In the letter to Menoeceus Epicurus is at pains to point out that the existence of chance does not militate against the achievement of the good life.  There is a random clement, an element of chance in nature, and Guyau was probably right in holding that Epicurus attributed it to the swerve of atoms."