“I am almost amazed that you consider a professional philosopher capable of no confusion in concepts and definitions. Such things are nowhere more at home than among philosophers who are not mathematicians, and Wolff was no mathematician, even though he made cheap compen- diums. Look around among the philosophers of today, among Schelling, Hegel, Nees von Esenbeck, and their like; doesn t your hair stand on end at their definitions? Read in the history of ancient philosophy what kinds of definitions the men of that day, Plato and others, gave (I except Aristotle). But even in Kant it is often not much better; in my opinion his distinction between analytic and synthetic theorems is such a one that either peters out in a triviality or is false.”

Attributed to Frege in: A. A. B. Aspeitia (2000), Mathematics as grammar: 'Grammar' in Wittgenstein's philosophy of mathematics during the Middle Period, Indiana University, p. 25

Source: Sanitary Economy (1850), p. 28-29
Context: It has been among the visions of some dreaming philosophers that human life is capable of almost indefinite extension. The great Condorcet was one of these. He thought that by the removal of the two causes of evil—poverty and superfluity—by destroying prejudices and superstitions, and by various other operations, which he considered the purification of mankind, but which other people would call their pollution, the approach of death would by degrees be farther and farther indefinitely protracted. It is desirable that the practical views entertained by sanitary reformers should be kept widely distinct from any such theories, the character of which has been well drawn by Malthus when he says—"... Though I may not be able in the present instance to mark the limit at which further improvement will stop I can very easily mention a point at which it will not arrive."

p, 125
Number: The Language of Science (1930)

The Usurpation Of Language (1910)
Context: We want philosophers, among other reasons, because the world is full of false philosophy. The way of experience is beset on every hand by a multitude of verbal judgments, of empty phrases, of word-copies, which pass themselves off as the real thing, which pretend to do duty for concrete fact and, by force of their number and importunity, capture our attention and cause the true originals to be overlooked. If it is true that philosophy must perforce fight its battles with words, is it not equally true that words are the weapons against which it must everywhere contend? The philosopher bent on the enlargement of experience perceives at once that his work cannot be done, cannot even be commenced, until he has cleared away the heaps of verbal detritus under which the bedrocks of experience lie buried.

1870s, On the Hypothesis that Animals are Automata, and Its History (1874)

trans. Michael Chase, p. 272
La Philosophie comme manière de vivre (2001)

I Want to be a Mathematician: An Automathography (1985)
Context: Mathematics is not a deductive science — that's a cliché. When you try to prove a theorem, you don't just list the hypotheses, and then start to reason. What you do is trial and error, experimentation, guesswork. You want to find out what the facts are, and what you do is in that respect similar to what a laboratory technician does. Possibly philosophers would look on us mathematicians the same way as we look on the technicians, if they dared.

Source: The Cambridge Companion to Newton, 2002, p. 1

Part 4, Section 7
A Treatise of Human Nature (1739-40), Book 1: Of the understanding
Context: This deficiency in our ideas is not, indeed, perceived in common life, nor are we sensible, that in the most usual conjunctions of cause and effect we are as ignorant of the ultimate principle, which binds them together, as in the most unusual and extraordinary. But this proceeds merely from an illusion of the imagination; and the question is, how far we ought to yield to these illusions. This question is very difficult, and reduces us to a very dangerous dilemma, whichever way we answer it. For if we assent to every trivial suggestion of the fancy; beside that these suggestions are often contrary to each other; they lead us into such errors, absurdities, and obscurities, that we must at last become asham'd of our credulity. Nothing is more dangerous to reason than the flights of the imagination, and nothing has been the occasion of more mistakes among philosophers. Men of bright fancies may in this respect be compar'd to those angels, whom the scripture represents as covering their eyes with their wings. This has already appear'd in so many instances, that we may spare ourselves the trouble of enlarging upon it any farther.