Kommentar zu Bereschit 15:18

לזרעך נתתי UNTO THY SEED HAVE I GIVEN — The promise of the Holy One, blessed be He, is as an accomplished fact. (Consequently the perfect tense is here used) (Genesis Rabbah 44:22).

IN THAT DAY THE ETERNAL MADE A COVENANT WITH ABRAM, SAYING. Now the Holy One, blessed be He, promised Abraham the gift of the land many times, and all of the promises served a purpose. When he originally arrived in the land, He said to him, Unto thy seed will I give this land,³³⁷Above, 12:7. but He did not clarify the extent of His gift, for included in this promise is only the land where he walked, unto the place of Shechem unto the oak of Moreh.³³⁸Ibid., Verse 6. Afterwards, when his merits increased while in the Land, He bestowed upon him the additional promise: Lift up now thine eyes, and look from the place where thou art, northward and southward, and eastward and westward,³³⁹Ibid., 13:14. meaning that He will give him all those lands in their totality, for the meaning of the expression, which thou seest,³⁴⁰Ibid., Verse 15. is not literally “with your eyes” for the sight of a person does not extend far. Rather, it means that He will give him [land which lies in] every direction in which he looks. It may be that He miraculously showed him all the land of Israel, as was the case with Moshe Rabbeinu.³⁴¹Deuteronomy 34:1-3. He further added in this second blessing: and to thy seed forever,³⁴⁰Ibid., Verse 15. and that his seed will increase as the dust of the earth.³⁴²Above, 13:16. At the third time, He clarified to him the boundaries of the land, mentioning all the ten nations [who presently inhabited it],³⁴³Here, Verses 19-21. and in addition He made a covenant with him that sin would not cause [the annulment of the gift]. When He commanded him concerning circumcision, He told him, for a possession forever,³⁴⁴Further, 17:8. that is to say, if they will be exiled from it they will again return and inherit it.³⁴⁵The promise, And to thy seed forever (13:15), does not imply that if they will be exiled from the land they will return and re-inherit it. It assures legal title but not necessarily actual possession. However, the expression, for ‘a possession’ forever, does indicate that it is to be their land forever. Hence even if they are exiled they will return and possess it. He also added at that time, And I will be their G-d,³⁴⁶See Ramban to Leviticus 18:25. meaning that He in His Glory will lead them, and they shall not be under the rule of a star or constellation or any power of the powers above, as will yet be explained in the Torah.³⁴⁶See Ramban to Leviticus 18:25.
Now at the time of the first gift, Scripture states, Unto thy seed will I give,³³⁷Above, 12:7. the verb being in a future tense, and similarly in the second time,³⁴⁷To thee will I give it. (Above, 13:15). because until then He had not given him the entire land, and therefore, He said to him, will I give it.³³⁷Above, 12:7. But at the third time, during the covenant, He said, Unto thy seed have I given,³⁴⁸In Verse 18 here. meaning that He will make the covenant for the gift that He had already given him. Similarly, at the time of the circumcision, when He said, for a possession forever,³⁴⁴Further, 17:8. He said to him, And I will give unto thee,³⁴⁴Further, 17:8. in the future tense.³⁴⁹Since, as explained above, this promises that if they will be exiled from the land, they will return and inherit it, the future tense refers to this future return to and repossession of the land.
Rashi wrote: Unto thy seed have I given.³⁴⁸In Verse 18 here. The word of the Supreme One is as if it were already accomplished. — But there is no need for this explanation in this passage.³⁵⁰Instead, the explanation is as set forth above.

ביום ההוא כרת ה׳ את אברם ברית. On that day G'd made a covenant with Abram. The reason that G'd used the past tense in referring to the gift of the land of Israel to Abraham and his descendants is that He had already made Abraham take possession of it, as we explained on 13,15. Abraham's children would henceforth own the land in their capacity as his heirs.