Omit "that which is." "Wisdom" is the subject of both clauses. She is "made nown," i. e., by the very force of contrast, in the midst of fools; or she is reserved and reticent in the one, noisy and boastful in the other. The Septuagint and some other versions get over the difficulty, by reading "Wisdom is not made known."

Wisdom resteth in the heart of him that hath understanding: but [that which is] (m) in the midst of fools is made known.
(m) Forasmuch as they are convicted by it, and silenced.

Wisdom resteth in the heart of him that hath understanding,.... It is in his heart, as the treasury where it is laid up, and where it is kept in safety; here it lies hid and undiscerned, unmolested and undisturbed; no noise is made about it, or any ostentation of it; it dwells quietly and constantly there;
but that which is in the midst of fools is made known; the least share of knowledge which such persons have, or think they have, does not lie long in the midst of them; they take every opportunity of showing it to others, or of letting others know what they have attained to; and thereby, instead of getting the character of wise and prudent men, obtain that of fools; for, though a prudent man is communicative of his knowledge to others, it is at proper times, and in proper places, and to proper persons, which fools do not observe; but, without any manner of judgment or discretion, or regard to persons, places, and seasons, vainly thrust out their knowledge, and so proclaim their folly. The Syriac version is,
"in the heart of fools it shall not be known;''
it has no place there.

Wisdom possesses the heart, and thus regulates the affections and tempers.

resteth--preserved in quietness for use, while fools blazon their folly (Proverbs 12:23; Proverbs 13:16).

33 Wisdom rests in the heart of the man of understanding;
But the heart of fools it maketh itself known.
Most interpreters know not what to make of the second line here. The lxx (and after it the Syr.), and as it appears, also Aquila and Theodotion, insert οὐ; the Targ. improves the Peshito, for it inserts אוּלת (so that Proverbs 12:23; Proverbs 13:16, and Proverbs 15:2 are related). And Abulwald explains: in the heart of fools it is lost; Euchel: it reels about; but these are imaginary interpretations resting on a misunderstanding of the passages, in which ידע means to come to feel, and הודיע to give to feel (to punish, correct). Kimchi rightly adheres to the one ascertained meaning of the words, according to which the Venet. μέσον δὲ ἀφρόνων γνωσθήσεται. So also the translation of Jerome: et indoctos quosque (quoque) erudiet, is formed, for he understands the "and is manifest among fools" (Luther) not merely, as C. B. Michaelis, after the saying: opposita juxta se posita magis elucescunt, but of a becoming manifest, which is salutary to these. Certainly בּקרב can mean among = in the circle, of Proverbs 15:31; but if, as here and e.g., Jeremiah 31:31, בקרב is interchanged with בלב, and if חכמה בקרב is the subject spoken of, as 1-Kings 3:28, then בקרב does not mean among (in the midst of), but in the heart of the fool. According to this, the Talmud rightly, by comparison with the current proverb (Meza 85b): אסתירא בלגינא קישׁ קישׁ קריא, a stater in a flaggon cries Kish, Kish, i.e., makes much clatter. In the heart of the understanding wisdom rests, i.e., remains silent and still, for the understanding feels himself personally happy in its possession, endeavours always the more to deepen it, and lets it operate within; on the contrary, wisdom in the heart of fools makes itself manifest: they are not able to keep to themselves the wisdom which they imagine they possess, or the portion of wisdom which is in reality theirs; but they think, as it is said in Persius: Scire tuum nihil est nisi scire hoc te sciat alter. They discredit and waste their little portion of wisdom (instead of thinking on its increase) by obtrusive ostentatious babbling.

Resteth - Is laid up and hid there. In the midst - In the heart. Made known - They will publish in all times and companies.