It was obvious from the very beginning that there were major differences between the twin boys Yaakov and Eisav. Not only were their physical characteristics dissimilar but their personalities could not have been more divergent. The Torah tells us that Eisav was an "איש ידע ציד איש שדה," "a man of the field," while Yaakov was an "איש תם ישב אהלים," "a scholarly man of the tents" (בראשית כ"ה:כ"ז). The differences between them continued to grow until Eisav actually wanted to kill his brother.
A pivotal episode in the relationship between Yaakov and Eisav had to do with the Berachos they received from their father Yitzchak after Rivkah disguised Yaakov so that he would receive the Beracha which Yitzchak had originally intended for Eisav. This unusual event raises a series of questions. Why did Yitzchak actually intend to give the Beracha to Eisav? Would Eisav have been the ancestor of the Jewish people if not for the deception of Rivkah? Why did Yitzchak and Rivkah not agree upon the recipient of the Berachos?
In order to answer these questions and understand this curious story, we must examine carefully the Berachos which Yitzchak was prepared to bestow. It is sometimes mistakenly assumed that Yaakov "stole" the Beracha of Avraham which had been given to Yitzchak and was intended for Eisav. This, however, is clearly not the case. The Beracha which Yitzchak intended for Eisav and went instead to the disguised Yaakov was the Beracha that Hashem should grant him the dew of Heaven and the fat of the earth, and much grain and wine (שם כ"ז:כ"א). The Beracha which Yitzchak gave to Yaakov when he knew it was Yaakov is found later, when Yitzchak says that Hashem should grant Avraham's blessing to him and his descendants, so that he will take over the land which Hashem gave to Avraham (שם כ"ח:כ'). The Torah makes it absolutely clear that the Beracha of Avraham was never intended for Eisav. Yitzchak knew all along that only Yaakov was fit to be the ancestor of the Jewish people. The Beracha meant for Eisav was another blessing entirely.
We thus see in this Parsha that Yitzchak actually had two Berachos to give - a physical one and a spiritual one. The latter was the Beracha of Avraham, and even Yitzchak (not only Rivkah) knew that this must be given to Yaakov. Yaakov alone possessed the spiritual qualities and character traits appropriate to carry on after Avraham and Yitzchak. The physical Beracha, granting "the fat of the earth..", was the subject of a dispute between Yitzchak and Rivkah; Yitzchak wanted this Beracha to go to Eisav while Rivkah felt that it too should go only to Yaakov.
Perhaps this difference of opinion between Yitzchak and Rivkah had to do with the personalities of the children. Yitzchak may have thought that Eisav, being the more physical of his sons, the "איש שדה", should receive the Beracha which grants physical riches. Eisav was a hunter whose concerns were primarily physical ones, with which Yitzchak wanted to help him. Rivkah, on the other hand, believed that to give such a blessing to a person like Eisav would be to drive him further and further away from spiritual pursuits. Furthermore, Eisav would be better able to take care of himself on his own. Yaakov, however, who may have neglected his physical needs since he was a "ישב אהלים", needed the physical assistance.
Our spiritual heritage was ensured by Yaakov Avinu who carried on the study of the Eternal truths passed to him by Avraham and Yitzchak. It was Rivkah Imeinu who saw to it that our physical sustenance would also be guaranteed by helping us receive that Beracha as well.