Your collection of philosophical papers I have perused with a great deal of pleasure. I take notice that several letters of mine to you make a part of it, which gave me an additional pleasure, as it places me in some respect in a situation I should always be glad personally to be in, near my friend Franklin.

There are several things in the collection, which I shall take the liberty to remark on, because by that means I have an opportunity of continuing a correspondence, which it will give me pleasure always to maintain.

I shall begin with the 16th. Letter p. 259 of your collection wherein you give a new and ingenious account why damp winds, though not colder by the thermometer, give a more uneasy sensation of cold than dry ones, viz. “because they conduct better; that is, are better fitted to convey away the heat from our bodies. The body cannot feel without itself; our sensation of cold is not in the air without the body, but in those parts of the body which have been deprived of their heat by the air.” Cold is here ascribed only to a negative cause, the absence or deprivation of heat. This may sometimes be the reason of our sensation of cold; but the same sensation may sometimes also arise from a positive cause, the accession of certain particles, which, by insinuating themselves into the blood, may interrupt its motion, and in that way give us the sensation of cold, which may be greater or less in proportion to the quantity of such particles. If cold was a mere negation of heat, I dont see why in the warmest seasons there should sometimes happen the most sudden transitions to cold.

Great heat and cold frequently succeed each other instanter: a flow of southern air, which has been greatly heated by the sun, brings along with it a great quantity of igneous particles, and causes hot weather; the air becomes considerably rarified thereby; the igneous particles (which are the cause of the rarefaction) act within a limited sphere, and, assisted by the elasticity of the rarefied air, bear up against the denser air, which is upon the borders of that sphere of action. So long as there is an exact equilibrium maintained between one and the other, they will remain at rest; but the equilibrium, which cannot be long supported by reason of the fluidity of the air, being destroyed, a current of denser air from the north takes place, and mixing with the rarer air, takes off part of its heat, and at the same time bringing certain nitrous particles along with it, communicates thereof to the rarer air, or the igneous particles may be conceived as ascending in consequence of their less specific gravity; which is manifestly the case when you pour cold water into hot you’ll be able to distinguish a greater heat on the surface than below. Hence a cold air proceeding from a positive cause; or from a positive and negative cause jointly; cold, I mean, as relative to our sensations: hence, also, (when the nitrous particles, or whatever be the positive cause, greatly abound,) snow, or hail; which may be called (as distinguished from our sensations) positive or absolute cold, or effects of a positive cause, which we denominate cold. The common experiment of freezing water before a fire by means of salt or nitre mixed with snow, seems to prove that cold proceeds from a positive cause, and is not a mere negation of heat.

In regard of our sensations, the same thing may be both hot and cold at the same time. If you hold one hand before a fire, and the other in snow, for some time, and afterward put them both into water of a middling degree of heat, you will feel the same water at the same time cold with one hand, and warm with the other; the water conducting the heat from the one, and communicating it (or some of its own) to the other, agreeable to your doctrine.

I am much pleased with your conjecture, p. 263, that there may be a region of electric fire above the atmosphere, and that the Aurorae Boreales may perhaps be currents of that fluid in its own region, becoming from their motion visible. It gives me a better opinion of the ancient philosophers than I ever had before, who, as you observe, thought there was a region of fire above the atmosphere. I join with you that we are but novices in this branch of natural knowledge, though I think you are making very happy advances out of your noviciate.

You seem to be p. 264, “much in the dark about light and are not satisfied with the doctrine that supposes particles of matter continually driven off from the sun’s surface with a swiftness so prodigious; for every particle with such a motion must (you think) do more execution than a 24 pounder discharged from a cannon, and yet it will not drive before it or remove the lightest dust.” This seems to be a weighty objection: but let us suppose a particle of light in motion to be o; then the momentum thereof, equal to its quantity of matter multiplied by its velocity, will be o: it can have no effect in that case on the smallest particle of dust to remove it. Now let us consider what reason there is to make such a supposition. In order to this I must refer you to my letter of November 12. 1753 copied in 42[d] page of your collection, where ’tis said that a point 1027709 million times less than of an inch affords light enough to affect the sight at four miles distance. If this calculation was just, it would show that the particles of light are inconceivably small; but upon a review I believe I did not take a right method of calculating, for I ought to have considered the light of the candle as filling a sphere (instead of a circle) of eight miles diameter, which I should have compared with a sphere of half an inch diameter, in order to come at the density of the light when just issuing from the candle (viz. at half an inch distance therefrom) and the density of it at four miles distance; in which case I should have found that, as those spheres are to each other as the cubes of their diameters, the density of the light within the smaller sphere would be 1041850618085376000 times greater than when it arrived at the larger sphere; and corresponding portions of those spheres being in the same proportion to each other as the spheres themselves, the light which would pass through the pupil of the eye (which is about of an inch diameter and would make a part of the large sphere) would be an equal number of times rarer than when it quitted the small sphere, and consequently must be contained, while within the small sphere, in a circle 1041850618085 million times less than of an inch diameter. Now on the supposition that such a point is possessed by a single particle of light, or that a single particle is equal thereto in bigness, I would ask whether the quantity of matter in such a particle would not be small in a greater degree, than its velocity (equal to that of the sun’s light) would be great? If so, a particle of light in motion may properly be supposed o, and its momentum not sufficient to remove the lightest dust, much less to do as much execution as a 24 pounder. We may go further to lessen the particles of light, by supposing that the candle light, viewed through a prism at the above-mentioned distance, would exhibit the same variety of colours (though not so vivid) as when viewed near; in which case the ray so viewed (which I have hitherto considered as a single particle) must be composed of a very great number of particles, which multiplied by the figures aforesaid would express the proportion that a particle of light is less than of an inch: But I have already got beyond my power of conception. I dont know whether this will be a sufficient answer to your objection, for I must acknowledge it does not altogether satisfy myself. If this objection was obviated, I think your other would not stand much in the way, viz. “That the sun must diminish exceedingly by such a waste of matter, and the planets recede to greater distances, through the lessened attraction; but neither is fact for aught we know.”

It has been apprehended, that the sun must diminish greatly by such a constant flux of light from its surface; and that the comets, which no other use could be found for, were intended to recruit it; if they recruit it, its attraction would not be lessened, nor, of consequence, the planets recede to greater distances. I shall acquiesce in this conjecture, till you apply the comets to a better use than the preservation of our Solar System, or perhaps of the universe.

I would now ask leave to consider your hypothesis p.265. You suppose “that all the phenomena of light may be more conveniently solved, by supposing universal space filled with a subtle elastic fluid, which when at rest is not visible, but whose vibrations affect that fine sense in the eye, as those of air do the grosser organs of the ear. We do not in the case of sound imagine that any sonorous particles are thrown off from a bell for instance, and fly in straight lines to the ear; why must we believe that luminous particles leave the sun and proceed to the eye.”

Diverse objections occur hereto, some arising from the supposition itself, and others from the comparison of light with sound. With regard to the first—if universal space be filled with a subtle elastic fluid, that fluid would always be at rest and therefore always invisible, and of consequence there would be always universal darkness; or, if any part could be put in motion, the whole must be in motion; for one single particle of it could not move without moving the next and that the next and so on ad infinitum; and in that case the least motion, let it commence when it would, must produce universal motion, and consequently universal light: and therefore there could be no medium between universal darkness and universal light. But if we relax the sense of the expression, and suppose universal space instead of being filled to abound greatly with the elastic fluid (in which sense I apprehend you intended to be understood) would not everything, which disturbed that fluid, cause a luminous appearance? Would not the inhabitants of the sea and air bespangle both, in all their motions, and thereby like prisms, exhibit the various colors according to the degree of vibration which those motions should occasion in the elastic fluid? and, as to ourselves, would not a radiance attend us wherever we went? What occasion should we have for candle light, when a quick vibration of the hand, or of machines easy to be invented, would dispel the night? Or rather might we not suppose there would be no night at all? For the action of the sun (if the sun should be necessary) would be communicated to us notwithstanding the interposition of the earth? And would not the effect of the sun’s action be only to enlighten us, unattended with heat, which enlivens and cherishes the universe? Would it be possible to collect and heat from the sun with glasses as we now do, if it only occasioned a vibration in the elastic fluid? and would not the elastic fluid, instead of exhibiting a round luminous body which we call the sun, be itself one continued universal blaze of light? These seem to be objections arising from the hypothesis: I shall mention several, which arise from the comparison above-mentioned. As sound (or a vibratory motion in the air which I would have considered here as synonymous) is propagated from the sonorous body in all directions, and surrounds and is propagated beyond or behind any obstacle in its way; so light, if it was a vibration of the elastic fluid, would surround and be propagated behind an obstacle, like sound: but this is not fact. Further, as sound confined for instance within the walls of an house, one of whose sides has an hole in it, would be propagated from that hole externally in circles, of which the hole would be the centre; so light, if it was a vibration, or was occasioned by a vibration of the elastic fluid, after passing through an hole would be propagated in circles, of which the hole would be the centre; but this is not fact; for light in its passage through the same uniform medium always passes in right lines. Besides, I am apprehensive that an objection somewhat similar to those above-mentioned against the common hypothesis, will lie against this; for the perpetual vibration, which the elastic fluid must be in, must keep light small bodies (and even a 24 pounder if suspended) in a perpetual vibratory motion, whereby the form and texture of those small bodies could not be duly examined, because the necessary means of such examination viz. a great deal of light would give them a proportionable vibration which would effectually prevent it. But we do not find any such motion, even in the lightest particles of dust; nor any difficulty arising therefrom, in such examination.

What you mention about the electrical spark that it is bright and visible at a distance, and this without fuel; and in such case “that no part of the electric fluid flies off in such case to distant places, but all goes directly, and is to [be] found in the place to which it is destined,” seems to favour your hypothesis. But may we not suppose that the electric fluid is heterogenous as well as light. The heterogeneousness of light is inferred from its colours, which are said to vary as the size of the particles vary: the variation becoming conspicuous by a prism and by other means, which class the particles according to their respective magnitudes, or degrees of refrangibility. Besides this which is the usual reason, another may be suggested from which the heterogeneousness of light may be inferred, namely, because it exhibits some effects similar to those of electricity: for example, a globe or pane of glass warmed in the sun or before a fire will attract and repel successively small cork balls, down, and such like bodies, properly circumstanced; and will shew other signs of electricity communicated to the glass by the sun or fire. So in regard to electricity its heterogeneousness may be collected from its producing some effects resembling those of light or fire, which are here considered as equivalent terms. Electricity and fire differ in many respects, and in some they agree as you have shewn in your letters on Electricity. So far as they agree in their effects, their natures may be presumed to be alike; or rather from this agreement or similitude of effects I would infer that they are mixed with and generally accompany each other; and that each produces its own effect at the time of their joint operation; the effects of electricity, similar to those of fire, being produced by the fire mixed with it; and the effects of fire, resembling those of electricity, being produced by the electricity mixed with that; the compound taking its name from the predominant principle. Thus, fire enflames bodies, and throws particles or light at a distance; hence the explosion of gun-powder and the luminous appearance occasioned by the electric spark; the fire mixed with it producing those effects. Thus, also, electricty attracts and repels light bodies alternately under given circumstances; hence the attraction and repulsion of glass heated before a fire; the electricity mixed with the fire producing those effects. In this way I would infer the heterogeneousness of light and electricity, and their mixture with each other; and in this way account for the similitude and difference of their effects; and for the luminous appearance of the electric spark in particular, without diminishing the pure electric fluid contained therein; no part of which (I am persuaded with you) flies off in the case you mention to distant places, but all goes directly, and is to be found in the place to which you destine it. On the same principles, the shining of diamonds in the dark, when rubbed and thereby electrified, may be accounted for without supposing that they lose any part of their matter. If no objection lay against your hypothesis, and the sun is not wasted by expense of light, I can easily conceive (in your method of accounting for it) that it shall otherwise always retain the same quantity of matter; but at present I cannot give into your “philosophical heresy”; for which, though you “are not subject to the Inquisition like poor Galileo,” you are not wholly without punishment, as it has occasioned you the trouble of this epistle. The last thing I shall take notice of at this time is your remarks on Baxter’s Vis Inertiae. You suppose there is no such property in matter, and I agree with you, if anything more be meant by it than a power of resisting, a power exactly equal to the quantum of its own force. Beyond that it is a mere chimera, as you have clearly proved; to which purpose your instance of two globes equal to the sun, set in motion by a musketo, is extremely pertinent. “Surely (to use your words) if it was anything more than a phantom, there might be enough of it in such vast bodies to annihilate by its opposition to motion so trifling a force.” I have no idea of it considered as a property or power: I understand it as expressing the absolute passiveness of matter; that when it is at rest, or in motion, it will continue so till acted upon by something without itself. Matter is commonly said to act upon, move, stop, resist matter; these expressions implying activity are apt to make us think, that matter has some power of action; but what appears is a mere effect, the effect of something distinct from itself. For example, I put the body A in motion, A moves B, B moves C; or rather A and B are the medium by which I move B and C. B, which is said to move C, is as much passive as A, which is immediately moved by me. I am the mover, they the moved; I am the agent, and they the patients. It is this passiveness, that the term vis inertiae (as I think) most properly expresses; though in any sense I apprehend it is used with great impropriety, it being in itself a solecism, a power of inactivity; that is, as power implies action, it must be a power of acting inactively. In regard to the power of resisting, in particular, which matter is said to be endued with, and which vis inertiae is commonly understood to signify, I conceive it to be a mere impression, or the continuation of an impression it originally received at its formation; or perhaps it is the effect of the perpetual action of some superior being upon matter.

Though I think you have demonstrated that vis inertiae, as considered above, is a nullity; yet I apprehend you are mistaken in some of your observations upon your author. You quote him thus “In No. 4, our author goes on and says, The body A requires a certain force to be impressed upon it, to be moved with a celerity as C, or such a force is necessary; and therefore it makes a certain resistance &c.; a body as 2A requires twice that force to be moved with the same celerity, or it makes twice that resistance, and so on.” This you think is not true, but “that the body 2A moved by the force 1F (though the eye may judge otherwise of it) does really move with the same celerity as 1A did when impelled by the same force: for 2A is compounded of 1A + 1A; and if each of the 1A’s, or each part of the compound were made to move with 1C (as they might be by 2F) then the whole would move with 2C, and not with 1C as our author supposes. But 1F applied to 2A’s makes each A move with ½C, and so the whole moves with 1C, exactly the same as 1A was made to do by 1F before.” You add “what is equal celerity but a measuring of the same space by moving bodies in the same time? Now if 1A impelled by 1F measures 100 yards in a minute; and in 2A impelled by 1F, each A measures 50 yards in minute which added make 100, are not the celerities as well as the forces equal?”

In answer to which I would observe, that the whole of it is grounded (as I apprehend) on a wrong supposition, viz. that the celerity of the compound differs from the celerity of its parts: but nothing is more clear to me than the contrary; for if the compound could have no celerity, when its parts were at rest (which I suppose will easily be granted) then, if its parts have a celerity, that celerity must be the celerity of the compound. The rest or celerity of its parts must be the rest or celerity of the compound; if it were otherwise, and your principles true, then 2A, which, by some force impressed upon it, moved 100 yards in a minute, would move with twice the celerity of 1A which moved 100 yds. in the same time: that is, 100 yards are twice so much as 100 yards. Or 2A and 1A being supposed to begin their motion from the same mark at the same time, and at the rate aforesaid, they would be both found at the end of a minute at 100 yards distance from the mark, and yet the largest would be 100 yards before the other. I can walk 12 miles in 4 hours at the rate of 3 miles per hour, but in this new way of calculating I should find I had walked in that time 24 miles; for each side of me, or each leg, would have gone 12 miles, which added to itself make 24 miles, being the number of miles both legs went; yet my body would be but 12 miles from the place where I began my walk, so that my legs would leave my body 12 miles behind them: or rather I should be at the same time in two places 12 miles apart; supposing my course direct. Your own definition of celerity will settle the point; “what is equal celerity (say you) but a measuring of the same space by moving bodies in the same time?” You make no distinction here with respect to the quantity of matter in the moving bodies; which should have been done in order to make the foregoing definition and principles compatible. You go on “Since force and celerity in the same quantity of matter are always in proportion to each other, why should we, when the quantity of matter is doubled, allow the force to continue unimpaired, and yet suppose one half of the celerity lost?” The reason I apprehend is (and it is a maxim as you observe) “that the force of bodies in motion is equal to the quantity of matter multiplied by the celerity.” If the celerity of a body be not altered, when its quantity of matter is doubled, its force will be doubled, by the foregoing maxim; and of consequence, in order that the force may continue unimpaired or unaltered, one half of its celerity, when its quantity of matter is doubled, must be lost. This maxim will serve also to discover the mistake (as I apprehend it) pointed out in the instances above quoted. I will apply it to one of them. “The body 2A moved by the force 1F does really (you say) move with the same celerity as 1A did when impelled by the same force.” Now let us suppose the celerity of 1A to be 2, then its force will be 2; the celerity of 2A being the same, viz. 2, its force will be 4, which is double the force of 1A: how are their forces to be made equal? No other way than by lessening the celerity of 2A, or increasing the celerity of 1A: if it be done by diminishing that of 2A, it must be diminished half; that is, its celerity instead of being 2 must be but 1: consequently the body 2A, moved by the force 1F, does not (as you suppose) move with the same celerity as 1A did, when impelled by the same force: and this is corroborated by sense: for to the eye (as you suggest) the body 2A would appear to move with less celerity than 1A.

At the end of your remarks you have started a thought that is new, relative to the formation of matter, and its capacity to act and think, viz. “If God was before all things, and filled all space; then when he formed what we call matter, he must have done it out of his own thinking immaterial substance. The same, though he had not filled all space; if it be true that ex nihilo nihil fit. From hence may we not draw this conclusion, that if any part of matter does not at present act and think, ’tis not from an incapacity in its nature, but from a positive restraint. I know not yet what other consequences may follow the admitting of this position, and therefore I will not be obliged to defend it.”

One of your premises, viz. “that God was before all things,” I acquiesce in; but the other “that he filled all space” must be qualified. If he filled all space in an absolute sense, ther could be no space for his creative power to exert itself in; or no space wherein the effects of that power could exist. This seems to be the sense you intended, and in that sense it appears to me your conclusion is necessarily deduced from the premises, viz. “that when God formed matter he must have done it out of his own thinking immaterial substance.” But God (as I conceive) did not fill all space absolutely, or in such a manner as to exclude other beings (the effects of his power) for other beings do exist: and they could not be formed out of his own thinking immaterial substance, for in that case, they could not be other beings, but a part of himself. Besides—may we not suppose that the divine substance, considered as distinct from matter, might pervade universal space, and that matter at the same time might exist therein, and not exclude the deity. Two substances even of the material kind may be conceived in some sort as possessing each other and the same space: as the electrical fluid and the body possessing it: so the substance of God may be conceived as possessing or pervading matter. I don’t mean to run any comparison between the nature of God and the electrical fluid; but only instance in that to assist my own conception. If it be possible, then, for matter to exist and not exclude the divine presence from the space it occupies, your conclusion, I think, is not a necessary one from the premises, for it is formed (if I mistake not) on this supposition, that the divine substance fills all space in such a manner as to exclude all other substances; from whence it would follow, if that supposition was true, and I think necessarily (as you seem to apprehend) “that when God formed what we call matter, he must have done it out of his own thinking immaterial substance.”

But you go on “The same [that is, the same conclusion would be just] though God had not filled all space; if it be true that ex nihilo nihil fit.” The most natural construction of this phrase, and which I think the most intelligible is, that nothing is produced without a cause; but this construction will not seem to support the aforesaid conclusion. I suppose then it means as applied to the case in hand, that something cannot be produced out of nothing by any being whatever. This being settled, the argument will stand thus—If no being whatever can produce anything out of nothing, then God when he formed matter must have done it out of his own thinking immaterial substance. I think this a necessary deduction; but I must beg leave to doubt of the truth of the proposition from whence it is made, for it stands alone, unsupported by any argument, and I cannot suggest any to myself in support of it; though I can several against it. We can judge of things only in proportion to our knowledge of them, and of consequence can form no adequate judgment of a power, whose extent we do not fully know. We cannot determine, then, even in regard to beings in a lower class than ourselves, precisely how far their powers extend; we only know in general that our own powers exceed theirs; and with respect to our own, which we must be most of all acquainted with, we do not fully know what they are capable of performing: many a man has done what he once thought himself not able to do and has failed in what he thought within the compass of his power. If we cannot determine the quantum of our own power, nor that of beings below ourselves much less can we of beings superior to us, and least of all, of the supreme being. In regard to the supreme being we are in some degree acquainted with his power—the more we inquire into it, the higher opinion we conceive of it—those, that know most of it, by being most acquainted with its effects, acknowledge that they know nothing of it in compare with what remains to be known: and they hesitate not to pronounce it infinite. That it is infinite may reasonably be supposed, but cannot (as I apprehend) be demonstrated: for the demonstrate it infinite, it must be demonstrated either, that its effects are infinite, or that all possible effects can be produced by it: but our demonstrations extend not to infinite, nor can relate to all possible effects.

Further—if no being could produce anything out of nothing, then God could not produce matter; but [it] is granted above that matter is produced and that God produced it. To this you will say, the matter produced, was produced out of his own thinking substance. I acknowledge it is difficult to conceive how anything can be produced out of nothing, and this difficulty is the principal reason, I suppose, why it is denied that any thing can: but is it not equally difficult to conceive how matter should be formed out of the divine substance which is as different from matter as their properties are different; as different and as much unlike as wisdom and figure, goodness and solidity, in which there is no more similitude between, than between matter and nonentity and therefore it is as difficult to conceive how matter can be formed out of the divine substance, as how it can be formed out of nothing; and if that difficulty be a sufficient reason to deny the one, it is a sufficient reason to deny the other. I shall next mention some consequences that flow from this position. If God formed matter out of his own thinking immaterial substance, then the substance of God must be diminished, and therefore he cannot be infinite: or if not diminished, the matter formed out of it must be God, or a part of God: in which case he must greatly debase and degrade himself, if matter be less excellent than thinking immaterial substance, and therefore he cannot be unchangeable: in which case he must be a compound of matter and spirit, and therefore not a pure and simple being: he must be both cause and effect, creator and creature, and therefore holy and unholy, wise and foolish, just and unjust, good and mischievous, eternal and temporal, unchangeable and mutable, almighty and frail, true and a liar, happy and miserable.

Lastly, if God formed matter out of his own thinking immaterial substance, he can’t be a necessarily existing being, for if he is, he must exist necessarily just as he is in every circumstance and mode of his existence; but such a change in his substance or essence is inconsistent with such necessary existence; and if he does not exist necessarily, he may not exist at all. If these consequences be just, the position aforesaid from whence they flow, viz. that God formed matter out of his own thinking substance, cannot be true; for so far as we have the means of judging, God is infinitely removed from such a character; and if that position be not true, the other viz. “Ex nihilo nihil fit” cannot be true; for matter, which it is granted had once no existence, must have been formed either out of the divine substance, or ex nihilo.

If the above reasoning be just (which you will be a better judge of than myself) does not your last conclusion viz. “that if any part of matter does not at present act and think, ’tis not from an incapacity in its nature, but from a positive restraint” appear unsupported? If it be capable of being supported, I know you can support it: but you are under no obligation to do it: for you declare you “know not what other consequences may follow from the aforesaid positions, and therefore will not be obliged to defend them.” For my own part, I am very little versed in metaphysics: but so far as I am able to judge, I am quite of your opinion “that there is great uncertainty in that science, and that the contradictions and disputes it affords, are endless:” and it is no wonder that this should be the case, for its object is beyond our ken.

I have great reason to ask pardon for trespassing so much upon your time and patience, and still greater for taking the freedom I have but your goodness exceeds all my trespasses, and, I make no doubt, will atone for them. Your collection of letters I returned to you a fortnight ago, which I hope have not miscarried.

I will give me great pleasure to hear your journey has been agreeable to you and that you are as well as I wish you. I am with the greatest esteem dear Sir your most obliged humble servant