e a fair right to regard as established truths, and
those which they know to be only more or less well-founded
speculations. Is any one prepared to deny that this is the first great
commandment of the ethics of teaching? Would any responsible
scientific teacher like to admit that he had not done his best to
separate facts from hypotheses in the minds of his hearers; and that
he had not made it his chief business to enable those whom he
instructs to judge the latter by their knowledge of the former?

More particularly does this obligation weigh upon those who address
the general public. It is indubitable, as Professor Virchow observes,
that "he who speaks to, or writes for, the public is doubly bound to
test the objective truth of that which he says." There is a sect of
scientific pharisees who thank God that they are not as those
publicans who address the public. If this sect includes anybody who
has attempted the business without failing in it, I suspect that he
must have given up keeping a conscience. For assuredly if a man of
science, addressing the public, bethinks him, as he ought to do, that
the obligation to be accurate--to say no more than he has warranty
for, without clearly marking off so much as is hypothetical--is far
heavier than if he were dealing with experts, he will find his task a
very admirable mental exercise. For my own part, I am inclined to
doubt whether there is any method of self-discipline better calculated
to clear up one's own ideas about a difficult subject, than that which
arises out of the effort to put them forth, with fulness and
precision, in language which all the world can understand. Sheridan
is said to have replied to some one who remarked on the easy flow of
his style, "Easy reading, sir, is--hard writing;" and any one who is
above the level of a scientific charlatan will know that easy speaking
is "----hard thinking."

Again, when Professor Virchow enlarges on the extreme incompleteness
of every man's knowledge beyond those provinces which he has made
his own (and he might well have added within these also), and when
he dilates on the inexpediency, in the interests of science, of
putting forth as ascertained truths propositions which the progress
of knowledge soon upsets--who will be disposed to gainsay him? Nor
have I, for one, anything but cordial assent to give to his
declaration, that the modern development of science is essentially
due to the constant encroachment of experi