Guide for the Perplexed, by Moses Maimonides, Friedländer tr. [1904], at sacred-texts.com
THE term ‘eẓeb is homonymous, denoting, in the first place, pain and trembling; comp. "In sorrow (be-‘eẓeb) thou shalt bring forth children" (Gen. iii. 16). Next it denotes anger; comp. "And his father had not made him angry (‘aẓabo) at any time" (1 Kings i. 6); "for he was angry (ne‘eẓab) for the sake of David" (1 Sam. xx. 34). The term signifies also provocation: comp. "They rebelled, and vexed (‘iẓẓebu) his holy spirit" (Isa. lxiii. 10); "and provoked (ya‘aẓibahu) him in the desert" (Ps. lxxviii. 40); "If there be any way of provocation (‘oẓeb) in me" (ib. cxxxix. 24); "Every day they rebel (ye‘aẓẓebu) against my words" (ib. lvi. 6).
In Genesis vi. 6 the word has either the second or the third signification. In the first case, the sense of the Hebrew va-yit‘aẓẓeb el libbo is "God was angry with them on account of the wickedness of their deeds" as to the words "to his heart" used here, and also in the history of Noah (ib. viii. 21) I will here explain what they mean. With regard to man, we use the expression "he said to himself," or "he said in his heart," in reference to a subject which he did not utter or communicate to any other person. Similarly the phrase "And God said in his heart," is used in reference to an act which God decreed without mentioning it to any prophet at the time the event took place according to the will of God. And a figure of this kind is admissible, since "the Torah speaketh in accordance with the language of man" (supra c. xxvi.). This is plain and clear. In the Pentateuch no distinct mention is made of a message sent to the wicked generation of the flood, cautioning or threatening them with death; therefore, it is said concerning them, that God was angry with them in His heart; likewise when He decreed that no flood should happen again, He did not tell a prophet to communicate it to others, and for that reason the words "in his heart" are added.
Taking the verb in the third signification, we explain the passage thus: "And man rebelled against God's will concerning him"; for leb (heart) also signifies "will," as we shall explain when treating of the homonymity of leb (heart).