As there has been a long truce between us since I last got a letter from you, and as I do not intend to despatch this   here till I receive Bob's answer with your address, I have no questions to answer, and any news would turn old by   keeping, so I intend briefly to state my country occupations (otherwise preparation for Cambridge, if you please).    I find that after breakfast is the best time for reading Greek and Latin, because if I read newspapers or any of  those things, then it is dissipation and ruin; and if I begin with props, experiments, or calculations, then I would be   continually returning on them. At first I had got pretty well accustomed to regular study with a Dictionary, and did  about 120 lines of Eurip. a day, namely, 40 revised, 40 for to-day, and 40 for to-morrow, with the looking up of    tomorrow's words. As I am blest with Dunbar's Lexicon, it is not very highly probable that I will find my word at   all; if I do, it is used in a different sense from Dunbar's (so much the better), and it has to be made out from the   context (either of the author or the Dictionary). So much for [139] regular study, which I have nearly forgot, for     when I had got to the end of the first chorus I began to think of the rods and wires that I had in a box. They have    entirely stopped Eurip., for I found that if I spent the best part of the day on him, and took reasonable exercise, I    could not much advance the making of the apparatus for tormenting these wires and rods. So the rods got the    better of the Lexicon. The observations on the rods are good for little till they are finished; they are of three kinds,     and are all distinguished for accuracy and agreement among themselves.

Thus—a rod bent by a weight at the middle takes the form of a curve, which is calculated to be one of the fourth   order. Let A C B be the rod bent by a weight at C. Mirrors fastened to it at A and B make known the changes of    the inclination of the tangent to the rod there, and a lens at C projects an image of a copper scale of inches and    parts from A to B, where it is observed, and so the deflection of the rod at C becomes known. Now the   calculated value of the elasticity deduced from the deflection differs from that deduced from the observations on   the mirrors by about 1/140 of either, and as the deflection at C was about ¼ inch, the difference of the observed   and calculated deflections is about 1/280 of an inch, which is near enough for home-made philosophical    instruments to go.
Thus you see I would run on about rods and wires, and weights, angles, and inches, and copper and iron, and  silvered glass, and all sorts of practicalities. Where is now Eurip.?—Ay, where? On the top of the Lexicon, and   behind bundles of observations and calculations. When will he come out? for he was a good soul after all, and    wise (beg his pardon, wiser). For the rest I have been at Shakespeare and Cowper. I used to put Thomson and  Cowper together (why?), and Thomson first; now they are reversed and far asunder.

 As I suppose any occupations are not very like yours, I pray you send me an account of what Oxford notions you  have got, either from Oxonians, books, or observation; and as, if I was to question you, you could but    answer my questions, I leave you to question yourself and send me some of the answers.

The only regular College science that I have thought of lately is Moral Philosophy. Whether it is an Oxford science I know not; but it must be, if not taught, at least interesting; so I purpose to fill up this letter with unuttered thoughts   (or cruder, which, as they are crammed into words, may appear like men new waked from sleep, who leap in   confusion into one another's breeches, hardly fit to be seen of decent men. Then think not my words mad if their  clothes fit them not, for they have not had an opportunity of trying them on before.

 There are some Moral Philosophers whose opinions are remarkable for their general truth and good sense, but not  for their utility, fixity, or novelty.   They tell you that in all your actions you ought to be virtuous, that benevolence is a virtue, that lawful rulers ought to be obeyed, that a man should give ear to his conscience.     Others tell you of unalterable laws of right and wrong, of Eternal truth and the Everlasting fitnesses of things.   Others of the duty of following nature, of every virtue between two vices (Aristot.), and of the golden mean. That   a man should do what is best on the whole (1) for himself; (2) for other men only, and not himself; (3) for the  whole universe, including himself, and so on. Now I think that the answers to the following questions should be  separate parts of M. Ph.:— -

1. What is man? This is the introduction, and is called statical or proper Metaphysics.

2. What are the laws of human action? Action being all that man does—thought, word, deed.

3. What are the motives of human actions?

4. What actions do men perform in preference to what others, and why?

5. What is the principle by which men judge some actions right, others wrong? [141]

6. What do particular men think of this principle? What are their doctrines?

7. What is the best criticism of right and wrong, or what (to us) is absolute right?

8. What are the best motives of human actions?

9. How are these motives to be implanted without violating the laws of human action?

10. What might, or rather what will, mankind become after this has been effected?

Moral Philosophy differs from Nat. Phil. in this, that the more new things we hear of in Nat. Phil. the better; but in   Mor. Phil. the old things are best, so that a common objection to Mor. Phil. is that everybody knows it all before.   If a man tells you that tyranny and anarchy are bad things, and that a just and lawful government is a good thing; it   sounds very fine, but only means that when men think the government bad from excess or defect they give it the   name of tyranny and anarchy. The ancient virtue of Tyrannicide was a man's determination to kill the king  whenever he displeased him. Thus it is easy to call a dog a bad name to beat him for. But there are other parts of  Mor. Phil. in which these are differences of opinion, such as the nature of selfishness, self-love, appetites, desires,     and affections, disinterestedness (what a word for a rush at!), which belong, to the first three questions, and so on.  I have told you something (p. 129) of three laws which I had been considering. In all parts of Mor. Phil. these   three laws seem to meet one, and in each system of Morals they take a different form. Now, that I might not  deceive myself in thinking that I was safe out of the hands of the philosophers who argue these matters, I have     been looking into the books of Moralists the most opposed to one another, to see what it is that makes them   differ, and wherein they agree. The three principles concerning, the nature of man are continually changing, their   shape, so that it is not easy to catch them in their best shape. Nevertheless:

 Lemma: Metaphysics.—A man thinks, feels, and wills, and therefore Metaphysicians give him the three faculties of  cognition, feeling, and conation. 

 Cognition is what is called Understanding, and is most thought of generally.

  Feelings are pleasures, pains, appetites, desires, aversions, approval and disapproval, love, hate, and all  affections.

 Conations are acts of will, whatever they be.

 Now to move a man's will it is necessary to move his affections. (How? Wait!) For no convictions of the    understanding will do, for a man does what he likes to do, not what he believes to be best for himself or others.    The feelings can only be moved by notions coming through the understanding, for cognition is the only inlet of    thoughts. Therefore, although it can be proved that self-love leads to all goodness, or, in other words, that   goodness is happiness, and self loves happiness, yet it can also be proved that men are not able to act rightly from     pure self-love; so that though self-love is a very fine theoretical principle, yet no man can keep it always in view, or    act reasonably upon it. Now, most moralists take for granted that the end which men, good or bad, pursue is their   own happiness, and that happiness, false or true, is the motive of every action, and that it is the only right motive.  Others say that benevolence is the only virtue, and that any action not done expressly for the good of others is   entitled to no praise.
Most of the ancients, and Hobbes among the moderns, are of the first opinion. Hutcheson and Brown (I think) are   of the second, and call the first selfish Philosophers and the selfish school. A few consider benevolence to the    whole universe as the proper motive of every action, but they all (says Macintosh) confound men's motives with   the criterion of right and wrong, the reason why a thing is right, and that which actually causes a man to do it. In   every book on Moral Philosophy some reference is made to that precept or maxim., which is declared to be the     spirit of the law and the prophets (see Matt. vii. 12), and the application of it is a good mark of the uppermost   thoughts or mode of thinking of the author.

Hobbes lays down as the first agreement of men to secure their safety, that a man should lay down so much [143]   of his natural liberty with respect to others, as he wishes that other men should to him. Hobbes having shown that    men, in what the poets and moralists call a state of nature (that is, of equality and liberty, and without government),    must be in a state of war, every man against every other, and therefore of danger to every man, deduces the  obligation of obeying, the powers that be from the necessity of Power to prevent universal war. Adam Smith's   theory of Moral Sentiments (which is the most systematic next to Hobbes) is that men desire others to sympathise   with them, and therefore do those things which may be sympathised with; that is, as Smith's opponents say, men   ought to be guided by the desire of esteem and sympathy. Not so. Smith does not leave us there, but I suppose    you have read him, as he is almost the only Scotch Moral Philosopher.

As it is Saturday night I will not write very much more. I was thinking to-day of the duties of [the] cognitive  faculty. It is universally admitted that duties are voluntary, and that the will governs understanding by giving or    withholding Attention. They say that Understanding ought to work by the rules of right reason. These rules are, or    ought to be, contained in Logic; but the actual science of Logic is conversant at present only with things either   certain, impossible, or entirely doubtful, none of which (fortunately) we have to reason on. Therefore the true   Logic for this world is the Calculus of Probabilities, which takes account of the magnitude of the probability (which   is, or which ought to be in a reasonable man's mind). This branch of Math., which is generally thought to favour   gambling, dicing, and wagering, and therefore highly immoral, is the only "Mathematics for Practical Men," as we    ought to be. Now, as human knowledge comes by the senses in such a way that the existence of things external is   only inferred from the harmonious (not similar) testimony of the different senses, Understanding, acting by the laws  of right reason, will assign to different truths (or facts, or testimonies, or what shall I call them) different degrees of    probability. Now, as the senses give new testimonies continually, and as no man ever detected in them any real    [144] inconsistency, it follows that the probability and credibility of their testimony is increasing day by day, and    the more man uses them the more he believes them. He believes them. What is believing? When the probability   (there is no better word found) in a man's mind of a certain proposition being true is greater than that of its being     false, he believes it with a proportion of faith corresponding to the probability, and this probability may be    increased or diminished by new facts. This is faith in general. When a man thinks he has enough of evidence for    some notion of his he sometimes refuses to listen to any additional evidence pro or con, saying, "It is a settled    question, probatis probata; it needs no evidence; it is certain." This is knowledge as distinguished from faith. He    says, "I do not believe; I know." " If any man thinketh that he knoweth, he knoweth yet nothing as he ought to  know.'' This knowledge is a, shutting, of one's ears to all arguments, and is the same as "Implicit faith" in one of its   meanings. "Childlike faith," confounded with it, is not credulity, for children are not credulous, but find out sooner    than some think that many men are liars. I must now to bed, so good night; only please to write when you get this,    if convenient, and state the probability of your coming here. We perhaps will be in Edinburgh when the Wise men   are there. Now you are invited in a corner of a letter by
                                    JAMES CLERK MAXWELL.