This section is named מקור הברכות.

On Genesis 26,12: ויזרע יצחק בארץ ההיא וימצא בשנה ההיא מאה שערים, ויברכהו ה', Bereshit Rabbah 64,6 comments and Rashi quotes that the meaning of מאה שערים is one hundred times what was reasonably to be expected. Rashi adds: "our sages say that this estimate was necessary in order to determine the amount of tithes to be given from that harvest."

The patriarchs observed all the commandments as revealed later on in the Torah. We know from Genesis 14,20 that Abraham tithed when he gave the tithes to Malki Tzedek. In our פרשה we have a passage showing that Isaac also gave tithes; we know, of course, that Jacob promised to give tithes from his specific statement וכל אשר תתן לי עשר אעשרנו לך, “And all that You will give me, I will surely tithe to You" (Genesis 28,22). Our sages in Midrash Hagadol on that verse say that Jacob even tithed people. They explain that if we include Ephrayim and Menasseh in the count, there should have been fourteen tribes. Deduct four, seeing that each wife had a first born who was already sanctified from birth, and you remain with ten. From these remaining ten Jacob set aside Levi as a tithe to G–d. You see that all the patriarchs observed the commandment בכל לבבם, בכל נפשם ובכל מאודם, "with all their hearts, their souls and their means." The words בכל מאודם can be understood literally, i.e. "a tithe of everything." The expression בכל נפשם refers to Jacob who even tithed נפשות, people; the expression בכל לבבם, refers to תרומה גדולה, something for which the Torah does not prescribe a fixed amount. The amount to be set aside for the priest is left to the generosity of the individual, i.e. it is something to be decided on by his heart, לבבו. Our sages (Terumah 4,3) describe one fortieth as the תרומה of a generous individual, one fiftieth as the average gift, and one sixtieth as the תרומה of a miser. They derive these details from the verse in Genesis 14,20. It would have sufficed for the Torah to say ויתן מעשר, that Abraham tithed. Why did the Torah have to add the word מכל, from all? They answer that this is an allusion to תרומה מעשר, the tithe that the Levite has to give to the priest from the מעשר ראשון, the regular tithe that he received from the Israelite. I will explain later how this is alluded to in the text. Abraham gave to Malki Tzedek both the regular tithe and the תרומת מעשר. The numerical value of the letters in the word כל, all, is 50 which is an allusion to the 2% the average person sets aside. [In order to appreciate the following calculation, one must assume that Abraham first divided the entire harvest into two equal parts. This is not mentioned by the author. Ed.] Abraham removed one tenth of the total [taking it from one half] as tithe. He also set aside one part in fifty from the same total amount (before having tithed it) to give it separately as תרומה גדולה. When you remove "ten" from fifty, you are left with forty. This means that Abraham gave one part in forty as תרומה גדולה. [The normal procedure is to remove the תרומה גדולה from the harvest before measuring the ten per cent tithe from the remaining 98% of the harvest. Ed] Even though the normal or average amount set aside for תרומה is one in 50= כל, Abraham was more generous than that. All the patriarchs acted in a similar fashion.

The commentary of Rashi above that the amount of Isaac's harvest is described in order to enable one to "correctly estimate" the tithes, appears to contradict the Halachah. We know that tithing must never be based on an estimate, but must be measured exactly (Maimonides Hilchot Maaser 1,14). Even if the amount tithed exceeds ten percent of the harvest, one did not fulfil the commandment if one has estimated instead of measured the required amount. We must therefore assume that Isaac measured the tithe, but added the תרומה גדולה by estimating the amount, and this is what the Midrash refers to. Similarly, the Mishna Terumot 1,7 says that one does not give תרומה by measure but by estimate from the total amount. The quantity of תרומה to be given as per our sages serves only as a guideline, not as absolute measurement. The scriptural allusion for this procedure is Numbers 18,27: ונחשב לכם תרומתכם, "your gift will be 'considered' for you." This means that the process depends on מחשבה, thought, rather than measure. Although the verse mentioned is written in connection with תרומת מעשר, we have a principle that if a statement is not needed where it appears we may apply it elsewhere, in this case we apply it to תרומה גדולה.

If you look closely you will find that what Rashi writes about "estimating," applies to all kinds of tithes, and not only to תרומה גדולה. Since Rashi writes that the estimate was based on how much the field was expected to yield, it appears that the method of estimating applied to all kinds of tithes, and not just to the מעשר ראשון. We may therefore conclude that already prior to the harvest Isaac "estimated" how much he would have to set aside as a tithe. Once the harvest was in the barn- at which time the obligation to give a tithe became operative, he measured carefully how much one tenth would amount to. Anything one eats from a crop before it is brought into the house or barn is not subject to tithes. Isaac, as we have shown, was especially careful to measure the grain even before anyone could have eaten from it. Since the verse under discussion deals with the commandment of tithing, it is appropriate for the Torah to add the words: "G–d blessed Him (Isaac)." We have a parallel verse in Malachi (3,10) when the prophet assures us הביאו את כל המעשר אל האוצר … והריקותי לכם ברכה עד בלי די, "Bring to Me all the tithes to the storage house,….and I will pour out for you endless blessings."

When I wrote that Abraham gave to Malki Tzedek both the מעשר and the תרומה, you may ask that one does not give מעשר to a priest but to a Levite, and that Malki Tzedek was a priest! The Arizal explains that Aaron was a re-incarnation of Malki Tzedek's soul, and that the prophet Samuel was a re-incarnation of Aaron's soul. This is hinted at by G–d when He said to Moses in Exodus 4,14: "Is there not Aaron your brother the Levite?" The prophet Samuel was a Levite, and Malki Tzedek was another name for שם the son of Noach. You will note that the first half of the name of the prophet שמואל is שם. Malki Tzedek, in addition to being a priest, as the Torah describes him in Genesis 14,18, was also a Levite, at least potentially.