2. The teacher should love his pupils--children, if he is called upon to teach children; young men, if he is to address young men. The teacher should not only think of the science he teaches, but of the fruits his pupils are to reap from it; one can only be interested in what he loves. A teacher indifferent toward the young, will never make the necessary effort to lead and educate them. 3. The teacher, in teaching, should unite in a just measure _discipline_ and _liberty_. Instruction naturally presupposes one that knows and one that does not know; and it is necessary that the one should direct the other; hence the necessity of discipline. But the purpose of instruction is to teach to do without the master--to be one's own master in thought and conduct; hence the necessity of liberty. This liberty should grow along with the instruction, and, of course, proportionately to age; but, at any age, one should take advantage of the faculties of a child, and make it as much as possible find out by itself what is within its reach. 4. The teacher should not separate _instruction_ from _education_. He should not only communicate knowledge--he should above all form men, characters, wills. Instruction is, besides, already in itself an education. Can one instruct without accustoming young minds to work, to obedience, to correct habits of thought; without putting into their hands good books; without giving them good examples? It is most true that one does not form men with pure and abstract science alone,--it is necessary to add the letters, history, morality, religion. The teacher, besides, should study the character of his pupils, should, through work and moral and physical exercises, put down presumption, correct unmanliness, combat selfishness, anticipate or restrain the passions. =109. Applied science--Industry--Medicine.=--Science may find its application in two ways, either to _things_, or to _men_. Applied to things, it is called _industry_; applied to men, _medicine_. There are no special duties concerning industrial pursuits. Engineers, private or in the service of the State, employed in civil or military works, have no other duties then the general duties of functionaries, military-men, employees, etc. It is not the same with medicine. There are here obligations of a special and graver nature. =110. Duties of the physician--His knowledge.=--Knowledge is an obligation in every profession; everywhere it is indispensable to know the thing one is engaged in; but, in medicine, ignorance is of a much more serious character: for it may end in _manslaughter_. How can any one attend the sick if he knows nothing of the human body; if he is ignorant of the symptoms of a disease? He has, it is true, the resource of doing nothing; but might not this also be manslaughter? Does he not then take the place of him who knows and might save the patient?