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more general law, which includes that par- | ticular law, and perhaps many others under it. This was all that Newton aimed at by his ether. He thought it possible, that, if there was such an ether, the gravitation of bodies, the reflection and refraction of the rays of light, and many other laws of nature, might be the necessary consequences of the elasticity and repelling force of the ether. But, supposing this ether to exist, its elasticity and repelling force must be considered as a law of nature; and the efficient cause of this elasticity would still have been latent.

11. Efficient causes, properly so called, are not within the sphere of natural philosophy. Its business is, from particular facts in the material world, to collect, by just induction, the laws that are general, and from these the more general, as far as we can go. And when this is done, natural philosophy has no more to do. It exhibits to our view the grand machine of the material world, analysed, as it were, and taken to pieces, with the connexions and dependencies of its several parts, and the laws of its several movements. It belongs to another branch of philosophy to consider whether this machine is the work of chance or of design, and whether of good or of bad design; whether there is not an intelligent first Mover who contrived the whole, and gives motion to the whole, according to the laws which the natural philosopher has discovered, or, perhaps, according to laws still more general, of which we can only discover some branches; and whether he does these things by his own hand, so to speak, or employs subordinate efficient causes to execute his purposes. These are very noble and important inquiries, but they do not belong to natural philosophy; nor can we proceed in them in the way of experiment and induction, the only instruments the natural philosopher uses in his researches.

12. Whether you call this branch of philosophy Natural Theology or Metaphysics, I care not; but I think it ought not to be confounded with Natural Philosophy; and neither of them with Mathematics. Let the mathematician demonstrate the relation of abstract quantity; the natural philosopher investigate the laws of the material system by induction; and the metaphysician, the final causes, and the efficient causes of what we see and what natural philosophy discovers in the world we live in.

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duction. In a word, final causes, good final causes, are seen plainly everywhere in the heavens and in the earth; in the constitu tion of every animal, and in our own constitution of body and of mind; and they are most worthy of observation, and have a charm in them that delights the soul.

14. As to Efficient Causes, I am afraid our faculties carry us but a very little way, and almost only to general conclusions. I hold it to be self-evident, that every production, and every change in nature, must have an efficient cause that has power to produce the effect; and that an effect which has the most manifest marks of intelligence, wisdom, and goodness, must have an intelligent, wise, and good efficient cause. From these, and some such self-evident truths, we may discover the principles of natural theology, and that the Deity is the first efficient cause of all nature. But how far he operates in nature immediately, or how far by the ministry of subordinate efficient causes, to which he has given power adequate to the task committed to them, I am afraid our reason is not able to discover, and we can do little else than conjecture. We are led by nature to believe ourselves to be the efficient causes of our own voluntary actions; and, from analogy, we judge the same of other intelligent beings. But with regard to the works of nature, I cannot recollect a single instance wherein I can say, with any degree of assurance, that such a thing is the efficient cause of such a phenomenon of nature.

15. Malebranche, and many of the Cartesians, ascribed all to the immediate operation of the Deity, except the determinations of the will of free agents. Leibnitz, and all his followers, maintain, that God finished his work at the creation, having endowed every creature and every individual particle of matter, with such internal powers as necessarily produce all its actions, motions, and changes, to the end of time. Others have held, that various intelligent beings, appointed by the Deity to their several departments, are the efficient causes of the various operations of nature. Others, that there are beings endowed with power without intelligence, which are the efficient causes in nature's operations; and they have given them the name of Plastic Powers, or Plastic Natures. A late author of your Lordship's acquaintance, has given it as ancient metaphysics, That every body in the universe is compounded of two substances united-to wit, an immaterial mind or soul, which, in the inanimate creation, has the power of motion without thought; and of inert matter as the other part. The celebrated Dr Priestley maintains, that

Lord Monboddo.-H.

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matter, properly organized, has not only the power of motion, but of thought and intelligence; and that a man is only a piece of matter properly organized.

16. Of all these systems about the efficient causes of the phenomena of nature, there is not one that, in my opinion, can be either proved or refuted from the principles of natural philosophy. They belong to metaphysics, and affect not natural philosophy, whether they be true or false. Some of them, I think, may be refuted upon metaphysical principles; but, as to the others, I can neither see such evidence for them or against them as determines my belief. They seem to me to be conjectures only about matters where we have not evidence; and, therefore, I must confess my ignor

ance.

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it a first principle, that every production requires active power, I can reason about an active power of that kind I am acquainted with-that is, such as supposes thought and choice, and is exerted by will. But, if there is anything in an unthinking inanimate being that can be called active power, I know not what it is, and cannot reason about it.

20. If you conceive that the activity of matter is directed by thought and will in matter, every particle of matter must know the situation and distance of every other particle within the planetary system; but this, I am apt to think, is not your Lordship's opinion.

21. I must therefore conclude, that this active power is guided in all its operations by some intelligent Being, who knows both the law of gravitation, and the distance and situation of every particle of matter with regard to every other particle, in all the changes that happen in the material world. I can only conceive two ways in which this particle of matter can be guided, in all the exertions of its active power, by an intelligent Being. Either it was formed, in its creation, upon a foreknowledge of all the situations it shall ever be in with respect to other particles, and had such an internal structure given it, as necessarily produces, in succession, all the motions, and tendencies to motion, it shall ever exert. This would make every particle of matter a ma

17. As to the point which gave occasion to this long detail, Whether there is reason to think that matter gravitates by an inherent power, and is the efficient cause of its own gravitation, I say, first, This is a metaphysical question, which concerns not natural philosophy, and can neither be proved nor refuted by any principle in that science. Natural philosophy informs us, that matter gravitates according to a certain law; and it says no more. Whether matter be active or passive in gravitation, cannot be determined by any experiment I can think of. If it should be said that we ought to conclude it to be active, because we per-chine or automaton, and every particle of a ceive no external cause of its gravitation, this argument, I fear, will go too far. Besides it is very weak, amounting only to this: I do not perceive such a thing, therefore it does not exist.

18. I never could see good reason to believe that matter has any active power at all. And, indeed, if it were evident that it has one, I think there could be no good reason assigned for not allowing it others. Your Lordship speaks of the power of resisting motion, and some others, as acknowledged active powers inherent in matter. As to the resistance to motion, and the continuance in motion, I never could satisfy myself whether these are not the necessary consequences of matter being inactive. If they imply activity, that may lie in some other cause.

19. I am not able to form any distinct conception of active power but such as I find in myself. I can only exert my active power by will, which supposes thought. It seems to me, that, if I was not conscious of activity in myself, I could never, from things I see about me, have had the conception or idea of active power. I see a succession of changes, but I see not the power, that is, the efficient cause of them; but, having got the notion of active power, from the consciousness of my own activity, and finding

different structure from every other particle in the universe. This is indeed the opinion of Leibnitz; but I am not prejudiced against it upon that account; I only wished to know whether your Lordship adopted it or not. Another way, and the only other way, in which I can conceive the active power of a particle of matter, guided by an intelligent Being, is by a continual influence exerted according to its situation and the situation of other particles. In this case, the particle would be guided as a horse is by his rider; and I think it would be improper to ascribe to it the power of gravitation. It has only the power of obeying its guide. Whether your Lordship chooses the first or the last in this alternative, I should be glad to know; or whether you can think of a third way better than either.

22. I will not add to the length of so immoderately long a letter by criticising upon the passages you quote from Newton. I have a great regard for his judgment; but where he differs from me, I think him wrong.

The idea of natural philosophy I have given in this letter, I think I had from him. If in scholia and queries he gives a range to his thoughts, and sometimes enters the regions of natural theology and metaphysics, this I think is very allowable, and is not to

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MY LORD,-To what cause is it owing that I differ so much from your Lordship in Physics, when we differ so little in Metaphysics? I am at a loss to account for this phenomenon. Whether is it owing to our having different conceptions to the same words?-or, as I rather think it is, to your being dissatisfied with the three general laws of motion ? Without them I know not indeed how to reason in physics. Archimedes reasoned from them both in mechanics and hydrostatics. Galileo, Huygens, Wren, Wallis, Mariotte, and many others, reasoned from them, without observing that they did so.

I have not indeed any scruples about the principles of hydrostatics. They seem to me to be the necessary consequences of the definition of a fluid, the three laws of motion, and the law of gravitation; and, therefore, I cannot assent to your Lordship's reasoning, either about the pressure of fluids, or about the suspension of the mercury in the barometer.

As to the first, the experiments which shew that fluids do, in fact, press undequaque, are so numerous, and so well known to your Lordship, that I apprehend it is not the fact you question, but the cause. You think that gravity is not the cause. Why? Because gravity gives to every part of the fluid a tendency downwards only; and what is true of every part, is true of the whole : therefore, the whole has no other tendency but downward. This argument is specious, but there is a fallacy in it. If the parts did not act upon one another, and counteract one another, the argument would be good; but the parts are so connected, that one cannot go down but another must go up, and, therefore, that very gravity which presses down one part presses up another: so that every part is pressed down by its own gravity, and pressed up, at the same time, by the gravity of other parts; and the contrary pressures being equal, it remains at rest.

This may be illustrated by a balance equilibrating by equal weights in both scales. I say each arm of the balance is equally pressed upwards and downwards at the same

time, and from that cause is at rest; although the tendency of the weights, in each of the scales, is downwards only. I prove it a posteriori; because the arm of a balance being moveable by the least force, if it was pressed in one direction only, it would move in that direction: but it does not move. I prove it a priori; because the necessary effect of pressing one arm down, is the pressing the other up with the same force : therefore, each arm is pressed down by the weight in its own scale, and equally pressed up by the weight in the other scale; and, being pressed with equal force in contrary directions, it remains at rest. Your Lordship will easily apply this reasoning to a fluid, every part of which is as moveable as the balance is about its fulcrum; and no one part can move, but an equal part must be moved in a contrary direction. And I think it is impossible we should differ in this, but in words.

Next, as to the barometer. You say the mercury is kept up by the expansive power of the air: but you say further, that it is not kept up by the weight of the air. I agree to the first, but not to the last. The expansive power of the air is owing to its being compressed; and it is compressed by the weight of the incumbent atmosphere. Its expansive force is exactly equal to the force that presses and condenses it; and that force is the weight of the air above it, to the top of the atmosphere-so that the expansive force of the air is the causa proxima, the weight of the atmosphere the causa remota of the suspension of the mercury. Your Lordship knows the maxim, Causa causæ est causa causati. The barometer, therefore, while it measures the expansive force of the air which presses upon the lower end of the tube, at the same time measures the weight of the atmosphere, which is the cause of that expansive force, and exactly equal to it. If the air was not pressed by the incumbent weight, it would expand in boundless space, until it had no more expansive force.

As to the observation in the postscript, it is true, that the gravity of the air, while it rests upon an unyielding bottom, will give no motion to it; but the mercury in the lower end of the tube yields to the pressure of the air upon it, until the weight of the mercury is balanced by the pressure of the air.

What your Lordship is pleased to call the Opus Magnum, goes on, but more slowly than I wish.-I am, most respectfully, my Lord, yours,

THO. REID.

VIII.

ON THE ACCELERATED MOTION OF FALLING

BODIES.

Glasgow College, Nov. 11, 1782. MY LORD,-My hope that your Lordship is in no worse state of health than when I left you, and that the rest of the good family are well, is confirmed by your continuing your favourite speculations. I promised to call upon you in the morning before I came away. I sent in Samuel to see if you was awake: he reported that you was sleeping sound; and I could not find it in my heart to disturb your repose.

When we say, that, in falling bodies, the space gone through is as the square of the velocity, it must be carefully observed that the velocity meant in this proposition, is the last velocity, which the body acquires only the last moment of its fall: but the space meant is the whole space gone through, from the beginning of its fall to the end.

inaccurate writer might confound with the last mentioned. It is this-that a body uniformly accelerated from a state of rest, will go through a space which is as the square of the last velocity. This is an abstract proposition, and has been mathematically demonstrated; and it may be made a step in the proof of the physical proposition." But the proof must be completed by shewing, that, in fact, bodies descending by gravitation are uniformly accelerated. This is sometimes shewn by a machine invented by S'Gravesande, to measure the velocities of falling bodies; sometimes it is proved by the experiments upon pendulums; and sometimes we deduce it by reasoning from the second law of motion, which we think is grounded on universal experience. So that the proof of the physical proposition always rests ultimately upon experience, and not solely upon mathematical demonstration.-I am, my Lord, respectfully yours, THO. REID.

IX.

AFTER THE DEATH OF HER HUSBAND,
LORD KAMES, IN 1782.

As this is the meaning of the proposition, your Lordship will easily perceive, that the velocity of the last moment must indeed EXTRACT OF A LETTER TO MRS DRUMMOND, correspond to the space gone through in that moment, but cannot correspond to the space gone through in any preceding moment, with a less velocity; and, consequently, cannot correspond to the whole space gone through in the last and all preceding moments taken together. You say very justly, that, whether the motion be equable or accelerated, the space gone through in any instant of time corresponds to the velocity in that instant. But it does not follow from this, that, in accelerated motion, the space gone through in many succeeding instants will correspond to the velocity of the last instant.

If any writer in physics has pretended to demonstrate mathematically this proposition-that a body falling by gravity in vacuo, goes through a space which is as the square of its last velocity; he must be one who writes without distinct conceptions, of which kind we have not a few.

The proposition is not mathematical, but physical. It admits not of demonstration, as your Lordship justly observes, but of proof by experiment, or reasoning grounded on experiment. There is, however, a mathematical proposition, which possibly an

I accept, dear madam, the present you sent me, as a testimony of your regard, and as a precious relic of a man whose talents I admired and whose virtues I honoured; a man who honoured me with a share of his conversation, and of his correspondence, which is my pride, and which gave me the best opportunity of knowing his real worth.

I have lost in him one of the greatest comforts of my life; but his remembrance will always be dear to me, and demand my best wishes and prayers for those whom he has left behind him.

When time has abated your just grief for the loss of such a husband, the recollection of his eminent talents, and of his public and domestic virtues, will pour balm into the wound. Friends are not lost who leave such a character behind them, and such an example to those who come after them.

A gold snuff box.

C.-LETTERS TO DR JAMES GREGORY.

I.

Glasgow College, April 7, 1783. DEAR SIR,-By favour of Mr Patrick Wilson, our Assistant Professor of Astronomy, I send you two more numbers of my lucubrations. I am not sure when I can send more, as I am not sure whether my scribe may soon leave the College.

I shall be much obliged to you if you will continue to favour me with your observations, though I have put off examining those you have sent until the MSS. be returned, which I expect about the end of this month, along with Dug. Stewart's observations. I have also sent the Genealogy of the Gregories, which your brother left with me: I suspected that it was more particular than the copy I had, but I find they agree perfectly.

You will please deliver it to him, with my compliments. The few days he was here he payed his respects to all the Professors and all his acquaintance, and they are all very much pleased with his appearance. If it please God to spare his life, I hope he will do honour to his Alma Mater, and to his friends.+

I know not upon what authority the Edinburgh and London news-writers have given contradictory accounts of Dr Hunter's settlements. There is nothing certainly known here. I know that, six or seven years ago, he made a settlement very favourable to this College. But whether this is altered, or in what respect, I believe nobody here knows. But we shall probably know soon. He was surely a man that did great honour to his country, and I doubt not but his publick spirit, which I take to have been great, will have disposed him to leave his books, medals, and other literary furniture-which he had collected at vast expense, and with great industry-in such a way as that it may be useful to the publick.

I beg you to make my best respects to Mrs Gregory, and to all your family; and I am, dear Sir,

Your most obedient Servant,
THO. REID.

His "Essays on the Intellectual Powers."-H. + This was the Rev. William Gregory, A. M. of Balliol College, Oxford, afterwards Rector of St Mary's, Bentham, and one of the Preachers of Canterbury Cathedral. He had studied at Glasgow previously to entering at Oxford.-H.

The celebrated Dr Wm. Hunter. He bequeathed his anatomical preparations, library, and collection of medals, to the University of Glasgow, and a sum of money for the erection of a museum.-H.

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I cannot get

more copied of my papers till next winter, and indeed have not much more ready. This parcel goes to page 658. I believe what you have got before may be one-half or more of all I intend. The materials of what is not yet ready for the copyer are partly discourses read in our Literary Society, partly notes of my Lectures.

Your judgment of what you have seen flatters me very much, and adds greatly to my own opinion of it, though authors seldom are deficient in a good opinion of their own works.

I am at a loss to express my obligations to you for the pains you have taken, and propose to take again upon it. I have carefully laid up the observations you sent me, to be considered when the copy they refer to is returned, and I hope for the continuation of them. The analogy between memory and prescience is, I believe, a notion of my own. But I shall be open to conviction on this and every thing else we may differ about.

I have often thought of what you propose -to give the History of the Ideal System; and what I have to say against it, by itself, and I am far from being positive that it stands in the most proper place. Perhaps it will be easier to judge of this when the work is concluded. I have endeavoured to

put it in separate chapters, whose titles may direct those who have no taste for it to pass over them. But I hope to have your opinion upon this point at more length when we meet. I observe that Boyle and others, who, at the Reformation of Natural Philosophy, gave new light, found it necessary to contrast their discoveries with the Aristotelian notions which then prevailed. We could now wish their works purged of the controversial part; but, perhaps, it was proper and necessary at the time they wrote, when men's minds were full of the old systems, and prepossessed in its favour. What I take to be the genuine philosophy of the human mind, is in so low a state, and has so many enemies, that, I apprehend those who would make any improvement in it must, for some time at least, build with one hand, and hold a weapon with the other.

I shall be very glad to see you here, and will take it as a favour if you acquaint me when you have fixed your time, that I may be sure to be at home. I beg you will

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