Yitzchak Avinu sensed a contradiction. The manner in which the “son” who stood before him spoke was gentle, pleasant and respectful. Hence, he assumed that it was Yaakov who stood before him. On the other hand, once he felt his hairy arms, he thought it was Eisav. Alternatively, the power of Yaakov’s voice was in his ability to plead with Hashem through the medium of prayer. Eisav was a “hands on” man; he lived with his hands – plundering and murdering. Nothing stood in the way of his hands. One question that weighs heavily on the reader: If Yitzchak questioned the identity of the son who stood before him; if he was uncertain whether it was Yaakov of Eisav, why did he acquiesce to give the blessing? Once he perceived that before him stood someone who appeared to be a fraud, comprised of Yaakov’s voice and Eisav’s hands, Yitzchak should have refused to bless this individual.
Horav Moshe Bick, zl, posits that Yitzchak loved Eisav. After all, he was his son – a son who had serious issues – but no less a son. Yitzchak was a firm believer in extracting the good from the bad, as Yirmiyah HaNavi states: V’im totzi yakar mizollel k’Fi tihiyeh, “If you bring forth an honorable person from a glutton, then you will be like My own mouth” (Yirmiyah 15:19). Hashem told Yirmiyah that if he succeeds in transforming figurative gluttons into righteous penitents, his wishes will be fulfilled. In other words, the concept of yakar mizollel exists; the right person under optimum circumstances can transform the life of an irreverent sinner. Yitzchak saw the concealed good within Eisav, and he was bent on gleaning it out. Yitzchak’s understanding of the concept of Sur meira v’asei tov; “Turn from evil and do good” (Tehillim 34:15), encouraged him to transform the evil into good. When Yitzchak observed Eisav performing the mitzvah of Kibbud Av, honoring his father, and meticulously separating maaser, tithing, he had hope for his errant son.
Thus, when Yaakov dressed in the guise of Eisav, and came before Yitzchak, the Patriarch thought that Eisav stood before him. The voice that seemed to be Yaakov’s was not a problem, because Yitzchak believed that Eisav had repaired his ways and was in the process of repenting. He was now a new person, Yaakov-like. When he heard the Name of Hashem emanating from Yaakov’s mouth, he thought that before him stood a fully-repentant Eisav.
As an aside, in the glossary to Rav Bick’s sefer, Chayei Moshe, the writer (not certain if this gloss is from Rav Bick or from the editor) explains that the concept of transforming bad to good is neither a simple process, nor is it appropriate for everyone. It means dealing directly and, at times, intimately with evil. This is a madreigah, spiritual plateau, that is not accessible to everyone. One who is not suitably prepared and spiritually-rooted can quite possibly sustain spiritual impairment. One should sufficiently distance himself from evil. Only the tzaddik gamur — complete, perfect, righteous person — who is sustained by the sitra d’yemina/kedushah, side of holiness, such as one who is a tzaddik ben tzaddik, whose father was also a righteous person, is permitted to attempt this challenge (of extracting yakar mizollel). This is in opposition to the sitra achara, “other” side, referring to the realm of evil or impure forces.
Thus, Yitzchak, who was a tzaddik ben tzaddik, could allow himself the luxury of reaching out to Eisav. Rivkah Imeinu, however, whose pedigree was on the murky side, was a tzadekes bas rasha; she focused on Yaakov and distanced herself from the evil represented by Eisav. This is alluded to when the Torah says: Va’yee’tar Yitzchak l’Hashem l’nochach ishto, “Yitzchak entreated Hashem opposite his wife” (Ibid. 25:21), which Rashi interprets to be in the sense that Yitzchak and Rivkah stood at opposite sides of the room when they prayed to Hashem. Now, we may understand their opposition differently. Yitzchak was from sitra d’yemina, and Rivkah was to his left in the sense that her lineage was not as spiritually refined as his. This all goes to show that not everything is as it seems. People have underlying reasons and motives which are often beyond our level of comprehension.