In this week’s Torah portion we read about the blessings that our patriarch Isaac gave to his son Jacob.
Let’s take a look at the the passage  in which Isaac talks about his old age and the fact that he did not know the day of his death as an introduction to his expressing his desire to bestow his blessings upon his firstborn son.
Rashi quotes the words “I do not know the day of my death” and comments: ‘Rabbi Yehoshua ben Korcha said: if one reaches the age of his parents he must worry five years before and five years after’. Isaac was 123 years old [at that time]. He said: perhaps I am destined to reach my mother’s age and she died at the age of one hundred and twenty-seven and I am already five years away from her age. So [he said] I don’t know the day I will die: maybe it will be [determined by] my mother’s age, maybe by my father’s age.”
We must understand:
Why is it necessary to explain what Isaac meant with his words “I don’t know the day of my death” if Isaac himself begins the sentence by saying “I have grown old”?
Rashi, as a rule, does not cite the sources or authors of his commentaries unless that information helps us understand the text. Why does Rashi cite the author, Rabbi Yehoshua ben Korcha, in this case?
In the previous portion, in reference to the text “and G-d blessed Isaac” , Rashi comments: “Abraham was afraid to bless Isaac because he foresaw that Esau would come out of him. He said, ‘Let the Master of the blessings come and bless whomever he pleases. G-d came and blessed [Isaac]”.
The question is this: the blessings Isaac received from G-d Himself, the “Master of blessings”, certainly included the blessing for long life. Why suspect that he was about to die at the age of one hundred and twenty-three, if his own parents, blessed by G-d, both lived longer than one hundred and twenty-three years?
It is for this reason that Rashi explains that the cause for Isaac’s concern was not his age in itself, but the fact that he was five years away from the age of his mother Sara when she passed away (one hundred and twenty seven). And if one were to ask how is it then that Isaac eventually lived to be one hundred and eighty years old? Rashi addresses that by saying that there are two references used to determine the limit of one’s life-span, the age of one’s mother and the age of one’s father. Indeed, the one hundred and eighty years that Isaac lived were five years more than the life-span of his father, Abraham.
One might ask:
Since by nature one’s life span is close to that of one of his parents, Isaac’s life span of one hundred and eighty years, five years longer than that of his father, Abraham, was merely one of the natural options. Where is the special blessing that Isaac received from G-d reflected in his life span?
Also: why did Isaac worry about his mortality at the age of one hundred and twenty-three, if that possibility was only based on natural considerations of nature? If he were to consider the blessing he received from G-d, shouldn’t he be confident that he would surely live much longer?
When Abraham and Sarah reached their ages of one hundred and ninety, respectively, the Torah  refers to them not only as “old”, but as “coming into their years”, that is, they were about to complete the extension of life that was designated for them, if it weren’t for the years added as a consequence of the special blessing that G-d gave them. It turns out, then, that Abraham’s natural life span was one hundred years and Sarah’s was ninety years.
According to this, by the time he was one hundred and five years old, five years older than his father Abraham’s natural life-span of one hundred years, Isaac had already lived the maximum life-span based on the natural life-span —sans the special blessings— of his parents.
According to this reasoning it turns out that the special blessing that Isaac received from the “Master of Blessings” did indeed add many years to his life; in fact, he received the same amount of years that the Divine blessing added to the life span of his father, Abraham. The total of one hundred and eighty years that Isaac lived implies seventy-five years more than the one hundred and five that would be his natural lifespan, taking into account his father’s natural lifespan of one hundred years, and that naturally one can live five years longer than his mother or father.
As to his concern, as he approached his mother’s age —that perhaps his life span was determined on the basis of his mother’s age— Isaac’s calculation was as follows: since the natural length of Sarah’s life was ninety years and after G-d’s special blessing she lived to be one hundred and twenty-seven, it turns out that the blessing had added thirty-seven years to her life. According to one of the natural possibilities that Isaac’s life-span could extend up to five years less than the total of his mother’s, the age of eighty-five would already have been his natural life span and the additional thirty-seven years with which his mother was blessed would have been his maximum extension (85 + 37 = 122); hence his concern.
The insightful student might still ask: Rashi explains earlier  that Abraham had passed away five years before his time so as not to suffer as a result of seeing the misconduct of his grandson Esau and thus the Divine blessing that “he would pass away in a ‘good old age'” was fulfilled . The implication is that in reality the years added to Abraham’s life as a result of the special blessing were really eighty years (from which five were deducted, as explained) and not seventy-five. According to this, if the years added to Yitzhak’s life as a consequence of the blessing were the same amount as that of his father, Avraham, shouldn’t he have lived 185 years (105+80) instead of only 180 (105+75)?
To clear up this doubt, Rashi quotes the author of the rule, Rabbi Yehoshua ben Korcha. The Talmud  relates that a number of elderly sages were asked regarding the secret of their long life, and documents what each of them answered. When Rabbi Yehoshua ben Korcha was asked about this, he replied, “I never gazed at the face of a wicked man”. According to this clarification by Rabbi Yehoshua ben Korcha we can understand that as a result of Isaac having gazed upon his son Esau’s face—even though he did not know that he was evil— some of the special length of his life (the five years more than his father’s life) was shortened. The impact of the special Divine blessing that Isaac received was that his life was extended by the same number of years that his father had actually received (seventy-five years).
The mystical dimension and practical application implicit in Rashi’s commentary:
The reason that Isaac gave so much importance to the minimum possibility of his life span, based on his mother’s life (rather than the maximum possibility based on his father’s), is because Isaac personified the quality of Gevurah, severity, and his behavior was defined by the criteria of judgment and demand. But he reserved that conduct only for himself, with regard to his own personal conduct. As far as others were concerned, however, the blessings he gave them were with utmost generosity, surpassing even the abundance of blessings that Jacob bestowed upon his sons and those with which Moses blessed the tribes.
The practical teaching here for each one of us, sons of Isaac, is the following: the criteria of Gevurah, severity and maximum demand, must be reserved and applied to one’s own conduct. Regarding others, one must apply the quality of Chessed, that is, love and acceptance, and give in a generous and unrestricted way, both in terms of the “dew of the heavens” as well as the “fat of the earth”.
Based on Likutei Sichot, Vol. 15, pp. 217-220