“And it came to pass, when Yitzhak was old and his eyes dimmed so that he could not see, that he called Esav his elder son and said to him: ‘My son,’ and he said to him: ‘Here I am.’ ... ‘Prepare for me delicacies such as I love, and bring them to me, that I may eat, so that my soul may bless you before I die.’” (Bereshit 27:1–4)
We must understand how Yitzhak could have intended to bless Esav and not Yaakov. Surely he saw that Yaakov was a wholesome man, dwelling in tents [of Torah study], and it would seem evident that he was the one worthy of receiving the blessings, not Esav.
However, we find in the Gemara (Kiddushin 39b) that Rabbi Yaakov at first believed that the reward for the mitzvot is received in this world, until he witnessed an incident that changed his view. A father instructed his son to fulfill the mitzvah of shiluach ha-ken —sending away the mother bird from the nest and taking the eggs or chicks — with the intent that his son would thereby merit the reward of two mitzvot for which the Torah promises long life: the mitzvah of honoring one’s father (by carrying out his command) and that of shiluach ha-ken. Yet, in the very act of fulfilling the mitzvah — when he sent away the mother bird and took the chicks — the son fell from the ladder and died. From this, Rabbi Yaakov concluded that the true reward for the mitzvot is reserved for the World to Come.
Thus we may say that Yitzhak believed that the reward for mitzvot is granted also in this world, for he saw that both he and his father Abraham were God-fearing and observant of His commandments, and yet they constantly lived in material well-being. Therefore, when he saw that Yaakov was a wholesome man devoted to Torah study, he assumed that surely he would be rewarded in this world as well, and did not require his blessings. Instead, Yitzhak wished to bless Esav - since he was a man of the field, not that much attached to Torah study - so that he too might have blessing and prosperity in this world.
[See there for a further and most delightful explanation that the HaKadosh Baruch Hu desired the blessings to come to Yaakov specifically through stratagem and subtlety.]
Zera Shimshon, Parashat Toldot, art. 16
