What is the meaning of "Tamim Tih'yeh ... "?
Rashi (citing the Sifri): It is to go with Hashem trustingly - not to search for the future 1 but to accept whatever happens in good faith. 2
Ramban (citing Targum Onkelos) and Targum Yonasan: You should be perfect in Yir'as Shamayim - unify your hearts (to focus exclusively) on Hashem, to believe that He alone does everything, that He alone knows the future and that one should seek the future only from Him or from His prophets or His pious men, but not from the celestial powers or from any other source. 3
Seforno: See second half of answer #2
Rashbam: To seek guidance from Hakadosh-Baruch-Hu and not from the dead.
Pesachim, 113b: It is a prohibition against consulting astrologers. 4
Sifri: It is a prohibition against consulting lotteries. 5
Seforno: Except by means of a Navi or of the Urim ve'Tumim (as prescribed by the Torah).
Rashi: Then you will belong to Hashem and to His portion.
Ramban: In fact, in the event that one does hear the future from any other source one should dismiss it and reassure oneself that 'Everything is in the Hands of Hashem', and will or will not happen, depending on our deeds and closeness to Hashem. See Ramban who elaborates further.
See Torah Temimah, note 66.
See Torah Temimah, note 67. See also Oznayim la'Torah, end of DH 'Tamim Tih'yeh' #4.
What are the implications of "Tamim Tih'yeh Im Hashem Elokecha"?
Oznayim la'Torah: It implies that when dealing with people, one should not be Tamim - but smart. 1
Oznayim la'Torah: As David ha'Melech wrote in Tehilim, 18:26 "Im Navar Tisbarar ve'Im Ikesh Titapol", and the Gemara in Megilah, 13b, quoting Ya'akov Avinu, states 'Achiv Ani be'Rama'us' (I am his [Lavan's] equal in trickery!').
