THE LETTER TO PYTHOCLES
The Letter to Pythocles briefly outlines Epicurus’ views on meteorology. Some people think it might not have been written by Epicurus himself but a follower. This is because the tone of the letter is slightly different to the letters to Herodotus and Menoeceus; specifically, it is fairly strident in places.  Some people dispute this, however, and say that the letter was written by Epicurus himself.  But written by Epicurus or not, it seems to accurately portray his views.
The first time you read the letter, it can be quite amusing. This sage, this philosopher, this man we go to for the answers to everything, doesn’t really answer anything at all in this letter. The whole piece can be captured by the following quote.
The turnings of the sun and moon could come to pass because of … it could also be because of … or because … or even because …
Phenomenon after phenomenon — clouds, lightning, the waxing and the waning of the moon — one by one are dealt with in this same fashion: ‘Well, it could be this… or maybe that… or neither… or both… or this other option… or something else I haven’t thought of’. But this turns out the be the brilliance of this letter. As Epicurus repeatedly says:
… For all such possibilities and those like them are in no way inconsistent with any of the clear facts, providing one always in such detailed enquiries keeps a firm hold on what is possible and can refer each of them to what is consistent with the phenomena …
As he also says in this letter:
For we should not do physics by following groundless postulates and stipulations, but in the manner called for by the phenomena … But when one accepts one theory and rejects another which is equally consistent with the phenomenon in question, it is clear that one has thereby blundered out of any sort of proper physics and fallen into mythology.
Or in other words… Do not be dogmatic. Observe the evidence. See what explanations fit the evidence. Do not reject any theory which does not contradict the evidence. Let the evidence speak for itself, and let yourself be open to hearing what the evidence has to say. And change your explanation when new evidence comes up!
Good thing, really, because even the brilliant science of Epicurus — one of the greatest thinkers in all human history  — has been superseded by the intervening twenty-three centuries of thought(!) But unlike many other philosophies or religions which lose their relevance as the science and world view they are founded on gives way, Epicurus’ world view remains relevant because his is predicated on a way of looking at the world, not specific physical doctrines, e.g., the world is the centre of the universe and so humans are specially favoured (e.g. Christianity). Well, actually, the ethics of Epicurus are profoundly derived from his atomist theory, but as Epicurus himself says, when it comes to certain things, such as the world being made of atoms, “… all such things … are consistent with the phenomenon in only one way. This is not the case with meteorological phenomena…” Or to put it another way, Epicurus recognised that ethics derive from a correct knowledge of the universe, and thus ethics are in some sense derived from a specific understanding of the universe; but in almost all respects, particular views about how the world works are not fundamentally important to ethics.
The Letter to Pythocles, by Epicurus,
Translated by Brad Inwood and L. P. Gerson
83. Epicurus to Pythocles, greetings:
84. Cleon delivered to me your letter, in which you continued to display a good will to us worthy of our concern for you and tried, not unconvincingly, to recall the lines of reasoning which contribute to a blessed life; and you requested that I send you a brief and concise [statement of our] reasoning concerning meteorological phenomena in order to facilitate your recollections. For our other writings on the topic are hard to recall, even though, as you said, you have them constantly in hand. We were pleased to receive this request from you and were seized by pleasant expectations. 85. Therefore, having written all the rest, we shall produce what you requested, since these lines of reasoning will be useful to many others too, and especially to those who have just begun to sample true physics and those who are entangled in preoccupations more profound than some of the general studies. So grasp them well and, holding them keenly in your memory, survey them in conjunction with the rest [of my summary of physics], which I sent to Herodotus as the Smaller Summary.
First of all, do not believe that there is any other goal to be achieved by the knowledge of meteorological phenomena, whether they are discussed in conjunction with [physics in general] or on their own, than freedom from disturbance and a secure conviction, just as with the rest [of physics]. 86. [Our aim is] neither to achieve the impossible, even by force, nor to maintain a theory which is in all respects similar either to our discussions on the ways of life or to our clarifications of other questions in physics, such as the thesis that the totality [of things] consists of bodies and intangible nature, and that the elements are atomic, and all such things as are consistent with the phenomena in only one way. This is not the case with meteorological phenomena, but rather these phenomena admit of several different explanations for their coming to be and several different accounts of their existence which are consistent with out sense-perceptions.
For we should not do physics by following groundless postulates and stipulations, but in the manner called for by the phenomena; 87. for our life does not now need irrationality and groundless opinion, but rather for us to live without tumult. And everything happens smoothly and (providing everything is clarified by the method of several different explanations) consistently with the phenomena, when one accepts appropriately what is plausibly said about them. But when one accepts one theory and rejects another which is equally consistent with the phenomenon in question, it is clear that one has thereby blundered out of any sort of proper physics and fallen into mythology. Some of the phenomena which are within our [experience] and are observed just as they really are do provide signs applicable to what comes to pass in meteorology, but we cannot observe meteorological phenomena; for they can occur in several different ways. 88. We must, however, observe the appearance of each thing and, with regard to the things connected with it, we must distinguish those whose coming to pass in several different ways is not testified against by what happens within our experience.
A cosmos is a circumscribed portion of the heavens which contains stars and an earth and all the phenomena, whose dissolution will involve the destruction of everything within it; it is separated off from the unlimited and terminates at a boundary which is either rare or dense; it is either revolving or stationary; it has an outline which is round or triangular, or some shape or other. For all of these are possibilities. For none of the phenomena in this cosmos testifies against [these possibilities], since here it is not possible to grasp a limit [of our cosmos].
89. It is possible to grasp that there is an unlimited number of such cosmoi; and that such a cosmos can come into existence both within a[nother] cosmos and in an intercosmos, which is what we call the interval between cosmoi, in a place containing much void and not in an extensive area which is completely void, as some people say; [this happens] when certain seeds of the right sort rush in from one cosmos or intercosmos — or even from several — [thereby] gradually causing conjunctions and articulations and movements to another place (if it so happens) and influxes from [atoms] which are in the right condition, until [the cosmos] is completed and achieves stability, [i.e.,] for as long as the foundations laid can accept additional material. 90. For one does not need just to have an aggregate come into being, or a rotation in the void in which a cosmos comes to be by necessity, as opinion holds, and [then] grows until it collides with another [cosmos], as one of the so-called physicists says. For this is in conflict with the phenomena.
The sun and the moon and the other heavenly bodies did not come into being on their own and then get included by the cosmos, but they immediately began to take shape and grow (and similarly for the earth and sea) by means of infusions and rotations of certain natures with fine parts, either breath-like or fiery or both. For sense-perception suggests that they [come into being] thus.
91. The size of the sun and the other heavenly bodies relative to us is just as big as it appears.  But relative to itself it is either bigger or a bit smaller than it is seen as being, or just the same size.  For in our experience too fire-signals, when seen from a distance, are observed in this way by our sense-perception. And every objection directed at this portion [of our theory] will be easily dissolved if one pays attention to the clear facts, which we set out in our book On Nature. 92. The risings and settings of the sun and the moon and the other heavenly bodies could occur by kindling and extinguishing, as long as the circumstances in both locales [i.e., east and west] are such as to produce the aforementioned events; for none of the appearances testifies against this. <And> they could also be produced by the appearance [of these bodies] above the earth and a subsequent blocking [by it]; for none of the appearances testifies against this either. And it is not impossible that their motions come to pass because of the rotation of the entire cosmos, or by its rest and their rotation, produced by the necessity generated when they [first] rose, at the beginning when the cosmos was [first] coming into being. [There is probably a lacuna here] 93. … by extreme heat produced by a certain kind of distribution of the fire which constantly impinges on the adjoining places.
The turnings of the sun and moon could come to pass because of the obliquity of the heaven, which is compelled in this way at [certain] times; similarly, it could also be because of the resistance in the air, or because the fuel which regularly fits their requirements is burned up or is insufficient in quantity; or even because these heavenly bodies had forced on them from the very beginning the sort of rotation which causes them to have a kind of spiral motion. For all such possibilities and those like them are in no way inconsistent with any of the clear facts, providing one always in such detailed enquiries keeps a firm hold on what is possible and can refer each of them to what is consistent with the phenomena, not fearing the slavish technicalities of the astronomers.
94. The waning of the moon and its subsequent waxing could come to pass by means of the turning of this body and just as well by means of the changing shapes of the air, and again, also because of the interposition [of other bodies], and in all the ways which the phenomena in our experience suggest for the explanation of this kind of thing–as long as one is not so enamoured of the method of unique explanations as to groundlessly reject the others, because of a failure to understand what it is possible for a man to understand and what is not, for this reason desiring to understand what cannot be understood. And again, it is possible that the moon produces its own light, and also possible that it receives it from the sun. 95. For in our own experience we see many things which produce their own light, and many which receive it from other things. And none of these meteorological phenomena is a hindrance [to these possibilities], as long as one remembers the method of several different explanations, considers together the hypotheses and explanations compatible with these, and does not, by looking to things which are not compatible, give them a pointless importance and so slide, in different ways on different occasions, into the method of unique explanations. And the appearance of a face in [the moon] could occur because of the variation among its parts, and because [some parts] are blocked, and by all the methods one might consider which are consistent with the phenomena. 96. For in the case of all the meteorological phenomena one must not give up tracking down such [posibiities]. For if one is in conflict with the clear facts, one will never be able to partake of genuine freedom from disturbance.
The eclipse of the sun and the moon could also come to pass by extinguishing, as is also observed to occur in our experience; and also by being blocked by certain other bodies, either the earth or the heavens or some other thing. And one should in this way consider the methods [of explanation] which are consistent with each other, and that it is not impossible that some of them may occur together. 
97. And again, we should grasp the orderliness of the cyclical periods [of the heavenly bodies] [as happening] in the same way that some of the things which also happen in our own experience [occur]; and let the nature of the divine not be brought to bear on this at all, but let it go on being thought of as free from burdensome service and as [living] in complete blessedness. For if this is not done, the entire study of the explanations for meteorological phenomena will be pointless, as it has already been for some who did not pursue the method of possible explanations and so were reduced to pointlessness because they thought that [the phenomena] only occurred in one manner and rejected all the other explanations which were also possible, and so were swept off into an unintelligible position and were unable to consider together the phenomena which one must accept as signs.
98. The varying lenghts of nights and days [could occur] as a result of the alternate swift and slow motions of the sun over the earth, <or even> as a result of covering the varying distances between places and certain places either faster or slower, as is observed [to happen] with some things in our experience; and we must speak in a manner consistent with these when we speak of meteorological phenomena. But those who accept one explanation are in conflict with the phenomena and have lost track of what it is possible for a man to understand.
Predictive weather signs could occur as a result of coincidental conjunctions of events, as in the case of animals which are evident in our experience, and also as a result of alterations and changes in the air. For both of these are not in conflict with the phenomena; 99. but it is not possible to see in what sort of cases the explanation is given by reference to this or that cause.
Clouds could come to be and to be formed both as a result of thickenings of air caused by the pressure of the winds, and as a result of the entanglements of atoms which grip one another and are suitable for producing this effect, and as a result of a collection of effluences from both earth and bodies of water; and it is not impossible that the formation of such compounds is also produced in several other ways. So rains [lit. waters] could be produced from the clouds, sometimes when they are compressed and sometimes when they undergo change; 100. and again, winds, by their egress from suitable places and motion through the air, [can cause rain] when there is a relatively forceful influx from certain aggregates which are suitable for such discharges.
Thunder can occur as a result of the confinement of wind in the hollows of the clouds, as happens in closed vessels [in] our [experience], and as a result of the booming of fire combined with wind inside the clouds, and as a result of the rupture and separation of clouds, and by the friction between clouds and their fragmentation when they have taken on an ice-like solidity. And the phenomena invite us to say that the entire topic as well as this part of it are subject to several different explanations.
101. And lightning flashes similarly occur in several different ways; for the [atomic] configuration which produces fire is squeezed out by the friction and collision of clouds and so generates a lightning flash; [it could] also [occur] as a result of the wind making the sort of bodies which cause this luminescence flash forth from the clouds; and by the squeezing of clouds when they are compressed, either by each other or by the winds; and by the inclusion [in them] of the light scattered from the heavenly bodies, which is then driven together by the motion of the clouds and winds and is expelled by the clouds; or as a result of the filtering of the finest form of light through the clouds  and as a result of its movement; and by the conflagration of the wind which occurs because of the vigour of its movement and its extreme compression; 102. and because the clouds are broken by the winds and the atoms which produce fire are then expelled and so produce the presentation of the lightning flash. And it will be easy to see [that it could happen] in a great many other ways, for him who clings always to the phenomena and who is able to contemplate together what is similar to the phenomena.
The lightning flash precedes the thunder in this sort of arrangement of clouds because the configuration which produces the lightning flash is expelled at the same time as the wind strikes [the cloud] and subsequently the wind, being confined, produces this booming noise; and because although both strike together, the lightning flash moves with a more vigorous speed towards us, 103. while the thunder comes later, just as happens with some things which strike blows and are observed from a distance.
Thunder bolts can occur as a result of repeated gatherings of winds, and their compression and powerful conflagration, and the fracture of one part and its very powerful expulsion towards the areas below, the breakage occurring because the places adjacent to it are more dense owing to the thickening of the clouds; and [it may occur] just as thunder too can occur, simply because of the expulsion of the fire, when a great deal of it is confined and very powerfully struck by the wind and has broken the cloud because it cannot escape to the adjacent areas since they are always compacting together.  104. And thunderbolts can be produced in several different ways — just be sure that myths are kept out of it! And they will be kept out of it if one follows rightly the appearances and takes them as signs of what is unobservable.
Whirlwinds can occur as a result of a cloud being forced in the form of a column downwards to regions below, being pushed by a mass of wind and driven by the power of the wind, while at the same time the wind outside pushes the cloud to one side; and by the formation of the wind into a circle when some air pushes down on it from above; and as a result of the compacting of the air around it, when a great flow of winds takes place and is not able to flow off to the side. 105. And when the whirlwind is forced down to the earth, tornadoes are produced, in whatever way their production might take place owing to the movement of the wind; and when it [is forced down] on the sea, waterspouts are produced.
It is possible that earthquakes occur as a result of the enclosure of wind in the earth and the juxtaposition of small masses [of wind?] with the earth and its constant movement, all of which produce the shaking in the earth. And [the earth] either takes this wind into itself from the outside or because solid blocks of earth fall inwards into cavernous places in the earth and turn the enclosed air into wind. <And> earthquakes may also be produced as a result of the mere transmission of the movement produced by the falling of many solid blocks of earth and the transmission [of this shock] back again when it collides with some more densely compressed parts of the earth. 106. And these movements of the earth may also occur in many other ways. [There may be a lacuna in the text here.]
And the winds happen to occur from time to time when on any occasion some foreign matter gradually enters in, and as a result of the collection of a tremendous amount of water; and the rest of the winds occur when even just a few fall into the many hollow spaces, if there occurs a transmission of their force.
Hail is produced by a quite powerful solidification, [a result of] a circular movement and [subsequent] division of certain breathlike particles; and also <because of> a more moderate solidification of certain watery particles <and> their simultaneous fracture, which at the same time condenses them and breaks them up, so that the solidified material forms compounds both within the distinct parts and in the aggregation. 107. It is not impossible that their circular shape is produced both because the extremities on all sides melt off and because, at the formation of the compound, [particles] (either watery or breathlike) surround it evenly, part by part on all sides, as is said.
Snow could be produced by the outpouring of fine [drops of] water from the clouds owing to the symmetry of the pores and to the constant and powerful friction on the right sort of clouds by the wind, followed by the solidification of this [water] during its movement as a result of some powerful conditions of coldness in the lower regions of the clouds. And as a result of this solidification in the clouds which have a uniform rareness this sort of outflow can also occur when the watery clouds rub against each other and lie side by side; and these cause a kind of compression and so produce hail — something which happens mostly in the spring. 108. And this aggregation of snow could also vibrate off when the clouds which have undergone solidification rub against each other. And it is also possible that snow is produced in other ways.
Dew is produced by the assembling from the air of [particles] which become productive of this sort of moisture; and also by an exhalation either from wet areas or areas which have bodies of water (which is the sort of place where dew is most likely to be produced) followed by their assembling in the same place and their production of moisture and finally by its movement to the lower regions, exactly as certain such things in our own experience <are observed being produced. And frost> 109. is produced <no differently> from dew, when certain such things are solidified in a certain way because of a certain condition of cold air.
Ice is produced both by the expulsion of the round configuration from the water and by the compression of the scalene and acute-angled [particles] which exist in the water; and also by the addition from the outside of such [particles], which are driven together and so produce solidification in the water by expelling a certain number of round [particles].
The rainbow occurs as a result of the sun shining on water-laden air; or as a result of some peculiar properties of these colours, either all [together] or one type at a time; and again, as a result of the reflection of this light the neighbouring regions of the air will take on the sort of coloration which we see because the sun shines on its parts. 110. This presentation of roundness occurs because the vision observes the distance as [being] equal from all directions, or [possibly] because the atoms in the air (or those in the clouds which are derived from the same air) are compressed in such a way that this compound gives off [the appearance of] roundness.
The halo around the moon is produced because air from all sides moves towards the moon; or when it evenly restricts [the movement of] the effluences sent off from it to such an extent that this cloudlike phenomenon forms around it in a circle and is not interrupted in the slightest extent; or it restricts [the movement of] the air around it symmetrically on all sides so that what is around it takes on a round and dense formation. 111. And this happens in certain parts either because a certain effluence forces its way in from the outside or because heat occupies passages suitable for the production of this effect.
Comets occur when, under suitable circumstances, fire is collected in certain places in the meteorological region at certain intervals of time; or when from time to time the heavens above us adopt a particular kind of movement, so that such heavenly bodies make their appearance; or the [comets] just rush in by themselves at certain times because of some circumstances and approach the regions where we happen to be and become prominently visible; and they disappear owing to opposite causes. 112. Certain heavenly bodies rotate in place [i.e., those near the pole, which never set], which occurs not only because that part of the cosmos around which the rest rotates is stationary, as some people say, but also because there is a circular rotation of air around it which prevents them from wandering around, as the other heavenly bodies do; or also because they do not have any appropriate fuel in adjacent regions, while there is [a supply of fuel] in the area where they are observed. And this [phenomenon] could also be produced in several other ways, provided one can reason out what is consistent with appearances.
The wandering of some of the heavenly bodies, if they really do happen to have this kind of movement, 113. and the regular motion of others could be a result of them starting out with circular movement and [then] having been forced in such a way that some of them move in the same uniform rotation while others move with a rotation which at the same time has certain irregularities; and it could also be that, according to the regions over which they move, in one place there are uniform regions of air which push them on continuously in the same direction and which burn uniformly, while elsewhere there are irregular [regions of air] of such a nature that the observed differences are produced. But to supply one cause for these facts, when the phenomena suggest that there are several different explanations, is the lunatic and inappropriate behaviour of those who are obsessed with a pointless [brand of] astronomy and of certain [others] who supply vain explanations, since they do not in any way liberate the divine nature from burdensome service. 114. That some heavenly bodies are observed being left behind by others occurs because although they move around in the same orbit they do so more slowly; and because they also move in the opposite direction being drawn backwards by the same rotation; and also because some rotate through a larger area and some through a smaller, though they turn with the same rotation. But to pronounce unqualifiedly on these matters is appropriate to those who wish [only] to make a display of wonders for the masses.
So-called falling stars could be produced in part by their own friction, and also because they fall wherever there is a massive outburst of wind, just as we said [occurred] in the case of lightning flashes; 115. also by a collection of atoms capable of producing fire, when similar material [congregates] to produce this result and also a motion where the surge produced by the original collection occurs; and also because wind is concentrated in certain dense and misty places and this ignites as result of its confinement, then breaks through the surrounding environment and is borne to the place to which the movement makes its surge; and there are other non-mythical ways in which this phenomenon could be produced.
The predictive weather signs which occur in certain animals occur by a coincidental conjunction of events; for the animals do not bring any necessity to bear on the production of winter, nor does any divine nature sit around waiting for these animals to come out [of hibernation] and [only] then fulfils these signs. 116. For such foolishness would not afflict any ordinary animal, even if it were a little more sophisticated, let alone one who possessed complete happiness.
Commit all of this to memory, Pythocles; for you will leave myth far behind you and will be able to see [the causes of the phenomena] similar to these. Most important, devote yourself to the contemplation of the basic principles [i.e., atoms] and the unlimited [i.e., void] and things related to them, and again [the contemplation] of the criteria and the feelings and the [goal] for sake of which we reason these things out. For if these things above all are contemplated together, they will make it easy for you to see the explanations of the detailed phenomena. For those who have not accepted these [ideas] with complete contentment could not do a good job of contemplating these things themselves, nor could they acquire the [goal] for the sake of which these things should be contemplated.
References and Notes
 Such as Brad Inwood & L. P. Gerson p.5 (1994) The Epicurus Reader: Selected Writings and Testimonia. Hackett Publishing Company, Inc.
 A lofty claim, but one which I feel is thoroughly justified due to his innovations.
 Scholiast: “This is also in book 11 of the On Nature; for, he says, if its size had been reduced because of the distance, its brightness would have been even more reduced; for there is no other distance more symmetrical with this [degree of brightness].”
 Scholiast: “But not as the same time.”
 Scholiast: “He says the same in book 12 of On Nature, and in addition that the sun is eclipsed by the fact that the moon darkens it, and the moon by the shadow of the earth, but also by its own retreat. this is also said by Diogenes the Epicurean in book 1 of his Selections.”
 Scholiast: “Or clouds were incinerated by the fire and the thunder is produced.”
 Scholiast: “It generally [strikes] on a high mountain, on which thunder bolts most often fall.”