What is the meaning of "Im Levavi"?
Rashi: It was unlike my mouth. The spies had a common counsel, and Kalev feared to tell them that he will not say like them (Radak - lest they kill him). When he came, he contradicted them [in front of Bnei Yisrael]. 1 "Ekev Haysah Ru'ach Acheres Imo" (Bamidbar 14:24). His heart was unlike his mouth!
Malbim: I did not report according to what my eyes saw, rather, like my heart understood. This was one of four reasons 2 why I deserved this reward (to receive Chevron).
Radak: He was able to do so because when he began to talk, they thought that he will say like them, and were quiet (refer to 13:30:2:1).
Malbim: The others are in verses 8-11. (a) There were enticers to sin, but I opposed them and did not fear the nation - "Achai? Himsiv Es Lev ha'Am v'Anochi Mileisi". (b) Moshe swore [that I will get it]. (c) Hashem's Hashgachah agrees to this - I am still alive and strong, 45 years later.
