Letter 95 — On the Usefulness of Basic Principles (§61)

“There are certain matters in philosophy which need admonition; there are others which need proof, and a great deal of proof, too, because they are complicated and can scarcely be made clear with the greatest care and the greatest dialectic skill. If proofs are necessary, so are doctrines; for doctrines deduce the truth by reasoning. Some matters are clear, and others are vague: those which the senses and the memory can embrace are clear; those which are outside their scope are vague. But reason is not satisfied by obvious facts; its higher and nobler function is to deal with hidden things. Hidden things need proof; proof cannot come without doctrines; therefore, doctrines are necessary.”