I wish I may be intelligible, and that I do not oppress your Lordship with the garrulity of old age. I find myself indeed growing

old, and have no right to plead exemption from the infirmities of that stage of life. For that reason, I have made choice of an assistant in my office. Yesterday, the College, at my desire, made choice of Mr Archibald Arthur, preacher, to be my assistant and successor *. I think I have done good service to the College by this, and procured some leisure to myself, though with a reduction of my finances. May your Lordship live long and happy. Yours,

. THO. REID. ¡

From Dr REID to Lord KAMES.

On the use of Conjectures and Hypotheses in Philosophical Investigation; and on the meaning of CAUSE when applied to Natural Philosophy.—The distinct Provinces of Physical and Metaphysical Reasoning pointed out.

1. I am now to answer the letter you honoured me with of 7th November. And first, I disclaim what you seem to impute to me,

to

* Mr Arthur, a man of learning, abilities and worth, filled the Chair of Moral Philosophy in the University of Glasgow for fifteen years, with a reputation which did not disappoint the hopes of his respectable predecessor. A volume of Discourses on Theological and Literary Subjects, which give a very favourable idea of his talents, the justness of his taste, and the rectitude of his moral and religious principles, has been published, since his death, by Professor Richardson of the same College,-a gentleman distinguished in the literary world; and who has done honour to the memory of his friend, by an interesting sketch of his Life and Character, subjoined to these Discourses.

to wit," the valuing myself upon my ignorance of the cause of gravity." To confess ignorance when one is conscious of it, I take to be a sign, not of pride, but of humility, and of that candour which becomes a philosoper; and so I meant it.

2. Your Lordship thinks, "That never to trust to hypotheses and "conjectures about the works of God, and being persuaded that "they are more like to be false than true, is a discouraging doc"trine, and damps the spirit of inquiry," &c. Now, my Lord, I have, ever since I was acquainted with Bacon and Newton, thought that this doctrine is the very key to Natural Philosophy, and the touchstone by which every thing that is legitimate and solid in that science, is to be distinguished from what is spurious and hollow; and I can hardly think, that we can differ in so capital a point, if we understood each other's meaning.

3. I would discourage no man from conjecturing, only I wish him not to take his conjectures for knowledge, or to expect that others should do so. Conjecturing may be a useful step even in natural philosophy. Thus, attending to such a phenomenon, I conjecture that it may be owing to such a cause. This may lead me to make the experiments or observations proper for discovering whether that is really the cause or not: And if I can discover, either that it is or is not, my knowledge is improved; and my conjecture was a step to that improvement. But, while I rest in my conjecture, my judgment remains in suspense, and all I can say is, it may be so, and it may be otherwise.

4. A cause that is conjectured ought to be such, that if it really does exist, it will produce the effect. If it have not this quality, it hardly deserves the name of a conjecture. Supposing it to have this quality, the question remains, Whether does it exist or not? And this, being a question of fact, is to be tried by positive evidence. h 2 Thus,

Thus, Des Cartes conjectured, that the planets are carried round thẹ sun in a vortex of subtile matter. The cause here assigned is sufficient to produce the effect. It may, therefore, be entitled to the name of a conjecture. But where is the evidence of the existence of such a vortex? If there be no evidence for it, even though there were none against it, it is a conjecture only, and ought to have no admittance into chaste natural philosophy.

5. All investigation of what we call the causes of natural phenomena, may be reduced to this syllogism, If such a caust exists, it will produce such a phenomenon: but that cause does exist: Therefore, &c. The first proposition is merely hypothetical. And a man in his closet, without consulting nature, may make a thousand such propositions, and connect them into a system; but this is only a system of hypotheses, conjectures or theories; and there cannot be one conclusion in natural philosophy drawn from it, until he conşults Nature, and discovers whether the causes he has conjectured do really exist. As far as he can shew that they do, he makes à real progress in the knowledge of nature, and not a step further. I hope in all this your Lordship will agree with me. But it remains to be considered how the second proposition of the syllogism is to be proved, to wit, that such a cause does really exist. Will nothing satisfy here but demonstration ?

senses.

6. I am so far from thinking so, my Lord, that I am persuaded we never can have demonstration in this case. All that we know of the material world, must be grounded on the testimony of our Our senses testify particular facts only: from these we collect, by induction, general facts, which we call Laws of Nature, or Natural Causes. Thus, ascending by a just and cautious induction, from what is less to what is more general, we discover, as far as we are able, natural causes, or laws of nature. This is the analytical part

part of natural philosophy. The synthetical part takes for granted, as principles, the causes discovered by induction, and from these explains or accounts for the phenomena which result from them. This analysis and synthesis make up the whole theory of natural philosophy. The practical part consists in applying the laws of nature to produce effects useful in life.

What is

7. From this view of natural philosophy, which I have learned from Newton, your Lordship will perceive, that no man who understands it, will pretend to demonstrate any of its principles. Nay, the most certain and best established of them may, for any thing we know, admit of exceptions. For instance, there is no principle in natural philosophy better established than the universal gravitation of matter. But, can this be demonstrated? By no means. the evidence of it then? It is collected by induction, partly from our daily experience, and from the experience of all nations, in all ages, in all places of earth, sea, and air, which we can reach; and partly from the observations and experiments of philosophers, which shew, that even air and smoke, and every body upon which experiments have been made, gravitate precisely in proportion to the quantity of matter; that the sea and earth gravitate towards the moon, and the moon towards them; that the planets and comets gravitate towards the sun, and towards one another, and the sun towards them. This is the sum of evidence; and it is as different from demonstration, on the one hand, as from conjecture, on the other. It is the same kind of evidence which we have, that fire will burn, and water drown, that bread will nourish, and arsenic poison, which I think would not properly be called conjecture.

8. It is proper here to explain what is meant by the cause of a phenomenon, when that word is used in natural philosophy. The word Cause is so ambiguous, that I fear many mistake its meaning,

and

and take it to mean the efficient cause, which I think it never does in this science.

9. By the cause of a phenomenon, nothing is meant but the law of nature, of which that phenomenon is an instance, or a necessary consequence. The cause of a body's falling to the ground is its gravity. But gravity is not an efficient cause, but a general law, that obtains in nature, of which law the fall of this body is a particular instance. The cause why a body projected moves in a parabola, is, that this motion is the necessary consequence of the projectile force and gravity united. But these are not efficient causes, they are only laws of nature. In natural philosophy, therefore, we seek only the general laws, according to which nature, works, and these we call the causes of what is done according to them. But such laws cannot be the efficient cause of any thing. They are only the rule according to which the efficient cause operates.

10. A natural philosopher may search after the cause of a law of nature: But this means no more than searching for a more general law, which includes that particular 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