“If a person a) is poorly, b) receives treatment intended to make him better, and c) gets better, no power of reasoning known to medical science can convince him that it may not have been the treatment that restored his health.”

Source: The Romantic Generation (1995), Ch. 7 : Chopin: From the Miniature Genre to the Sublime Style

Power: A Radical View (1974)

Letter to Marin Mersenne (1637) as quoted by D. E. Smith & M. L. Latham Tr. The Geometry of René Descartes (1925)

Source: The Twelve Caesars, Gaius Caligula, Ch. 10

" Full Remarks: Governor Larry Hogan Announces Cancer Diagnosis http://governor.maryland.gov/2015/06/22/full-remarks-governor-larry-hogan-announces-cancer-diagnosis/"(22 June 2015)

Lecture IX : On the Conduct of the Understanding.; this provides the origin of the phrase "a square peg in a round hole".
Elementary Sketches of Moral Philosophy (1849)
Context: It is a very wise rule in the conduct of the understanding, to acquire early a correct notion of your own peculiar constitution of mind, and to become well acquainted, as a physician would say, with your idiosyncrasy. Are you an acute man, and see sharply for small distances? or are you a comprehensive man, and able to take in, wide and extensive views into your mind? Does your mind turn its ideas into wit? or are you apt to take a common-sense view of the objects presented to you? Have you an exuberant imagination, or a correct judgment? Are you quick, or slow? accurate, or hasty? a great reader, or a great thinker? It is a prodigious point gained if any man can find out where his powers lie, and what are his deficiencies, — if he can contrive to ascertain what Nature intended him for: and such are the changes and chances of the world, and so difficult is it to ascertain our own understandings, or those of others, that most things are done by persons who could have done something else better. If you choose to represent the various parts in life by holes upon a table, of different shapes, — some circular, some triangular, some square, some oblong, — and the persons acting these parts by bits of wood of similar shapes, we shall generally find that the triangular person has got into the square hole, the oblong into the triangular, and a square person has squeezed himself into the round hole. The officer and the office, the doer and the thing done, seldom fit so exactly, that we can say they were almost made for each other.

Source: 1880s, Incidents and Anecdotes of the Civil War (1885), pp. 317–318

1870s, Second Inaugural Address (1873)

Variant translation:
It is very important to give advice to a man to help him mend his ways. It is a compassionate and important duty. However, it is extremely difficult to comprehend how this advice should be given. It is easy to recognise the good and bad points in others. Generally it is considered a kindness in helping people with things they hate or find difficult to say. However, one impracticality is that if people do not take in this advice they will think that there is nothing they should change. The same applies when we try to create shame in others by speaking badly of them. It seems outwardly that we are just complaining about them. One must get to know the person in question. Keep after him and get him to put his trust in you. Find out what interests he has. When you write to him or before you part company, you should express concrete examples of your own faults and get him to recall to mind whether or not he has the same problems. Also positively praise his qualities. It is important that he takes in your comments like a man thirsting for water. It is difficult to give such advice. We cannot easily correct our defects and weak points as they are dyed deeply within us. I have had bitter experience of this.
Hagakure (c. 1716)
Context: To give a person one's opinion and correct his faults is an important thing. It is compassionate and comes first in matters of service. But the way of doing this is extremely difficult. To discover the good and bad points of a person is an easy thing, and to give an opinion concerning them is easy, too. For the most part, people think that they are being kind by saying the things that others find distasteful or difficult to say. But if it is not received well, they think that there is nothing more to be done. This is completely worthless. It is the same as bringing shame to a person by slandering him. It is nothing more than getting it off one's chest.
To give a person an opinion one must first judge well whether that person is of the disposition to receive it or not. One must become close with him and make sure that he continually trusts one's word. Approaching subjects that are dear to him, seek the best way to speak and to be well understood. Judge the occasion, and determine whether it is better by letter or at the time of leave-taking. Praise his good points and use every device to encourage him, perhaps by talking about one's own faults without touching on his, but so that they will occur to him. Have him receive this in the way that a man would drink water when his throat is dry, and it will be an opinion that will correct faults.
This is extremely difficult. If a person's fault is a habit of some years prior, by and large it won't be remedied. I have had this experience myself. To be intimate with all one's comrades, correcting each other's faults, and being of one mind to be of use to the master is the great compassion of a retainer. By bringing shame to a person, how could one expect to make him a better man?