Kommentar zu Bereschit 15:8

How shall I know: Our Rabbis, may their memory be blessed, said (Nedarim 32a), "Because Avram said, 'How shall I know,' he was punished with, 'You shall surely know [that your offspring shall be strangers in a land not theirs, and they shall be enslaved and oppressed four hundred years]' (Genesis 15:13)." But the intellect demurs from accepting this drash (homiletical understanding): That his children should suffer such a great punishment because of the sign that Avram requested; and Avram himself did not receive any punishment, whereas his children's teeth were set on edge. Therefore my heart tells me and has concluded that the exile in Egypt had other causes. And our Rabbis, may their memory be blessed, disagree about [the causes] – and you can find all of the opinions in [the commentary on the Torah] of our teacher, Rabbi Yitzchak Abarbanel, as he gathered them together. But (the reason for not quoting them here is that) the length of a page is too short to put all of those opinions upon it. Rather the writer of this drash was bothered by [the following question]: Whatever the reason for the exile may have been – why is it that the Holy One, blessed be He, should tell Avram this bad news, to distress him for nothing? It is about this that he said that [it was] for the sin of "How shall I know' – that he wanted to know something that was unnecessary to know, as why did he need to ask for a sign about the word of God? [Hence] he was punished with, 'You shall surely know,' such that the Holy One, blessed be He, informed him about something to distress him. And this is also poetic justice (middah keneged middah).

במה אדע, for possibly my descendants will sin and forfeit their claim to this land.

?ויאמר.. במה אדע כי אירשנה, he meant: “how do I know that my son will inherit it?” We have a similar construction in Genesis 48,22 אשר לקחתי מיד האמורי, where Yaakov also does not mean that he, personally, had battled the Emorite, but that his sons had done so successfully. The meaning of the words במה אדע cannot be that the same man who had just been given credit for his utter faith in G’d now has developed doubts, nothing having occurred to cause such doubts. He wanted to know how he could be certain that when his sons, i.e. offspring, would inherit the land that it would remain theirs forever. Perhaps, due to some sin, future generations might forfeit their claim to the land of Canaan, just as the present occupants had forfeited their claim through their sins. Unless they had, why would G’d want to dispossess them? He hoped that just as G’d had shown him the stars as an illustration that his offspring would be people of great substance, so G’d would show him a further illustration of a means by which his offspring would reinforce their title to that land once they had settled in it.