Midrash su Deuteronomio 8:19

(Ibid. 19:5) "And now, if you hearken to My voice": Take it upon yourselves now, all beginnings being difficult. "If hearken you shall hearken": From here it was derived: If a man hearkened to one mitzvah, he is caused to hearken to many mitzvoth. If he forgot one mitzvah, he is caused to forget many mitzvoth, viz. (Devarim 8:19) "If forget you shall forget, etc." "and you keep My covenant": the covenant of Torah. R. Akiva says: the covenant of circumcision.

And He said: “If thou wilt diligently hearken to the voice of the Lord thy God” (Exod. 15:26). Because of this verse they said: The man who hearkens to God’s word in time will hearken at other times as well, but if he ignores it at one time, he will do likewise at other times. Scripture teaches this in the verses And it shall come to pass, if ye diligently hearken (Jer. 17:24), and It shall be, too, if thou shalt forget (Deut. 8:19). How is that? If a man desires to hear only that which is to his advantage, he will hear that which is not to his advantage as well; and if he wishes to forget when it is to his advantage to do so, he will forget even when it is not to his advantage to do so. The power to choose is given to man. If it concerneth the scorners, He scorneth them, but unto the humble He giveth grace (Prov. 3:34). If a man hearkens to one command, he will be caused to hearken to many; but if he desires to forget even one commandment, he will be caused to forget many. Therefore it is written: If thou shalt begin to forget, thou wilt in time forget. Others reach this conclusion from the verse If thou at all take thy neighbor’s garment to pledge (Exod. 22:25); that is, if you should seize one garment as a pledge, you will ultimately take many pledges because of it.

(Exodus 15:26) "And He said: If pay heed, you shall pay heed": From here it was derived: If a man paid heed to one mitzvah, he is caused to pay heed to many mitzvoth, it being written "If pay heed, you shall pay heed." If a man forgot one mitzvah, he is caused to forget many mitzvoth, viz. (Devarim 8:19) "If forget, you shall forget." "to the voice of the L rd": This refers to the Ten Commandments, which were from Mouth to mouth through the medium of "voices." "and you do what is just in His eyes": These are select aggadoth, which are "heard" in the ears of all men. "and you give ear to His mitzvoth" — Mishnayoth. "and you keep all His statutes" — halachoth. "all of the illness which I placed in Egypt I will not place upon you." But if I do place it upon you — "for I am the L rd who heals you." These are the words of R. Yehoshua. R. Elazar Hamodai says: "hear": I might think that this is optional; it is, therefore, written "you shall hear": It is mandatory and not optional. "you shall pay heed": This is the rule which is the underpinning of Torah. "to the voice of the L rd your G d": We are hereby apprised that if one hears (Torah) from the mouth of his teacher, it is accounted to him as if he stood and paid attendance upon Him who lives and endures forever and ever. "and you do what is just in His eyes": This refers to (integrity) in one's dealings (with others). We are hereby apprised that if one deals faithfully (with others), men take pleasure in him, and it is accounted to him as if he fulfilled the entire Torah. "and you give ear to His mitzvoth" — laws. "and you keep all His statutes" — (those pertaining to) illicit relations. "all of the illness which I placed in Egypt I will not place upon you": And what is the intent of "for I am the L rd who heals you"? The Holy One Blessed be He said to Moses: Tell Israel that the words of Torah which I gave to you are healing for you, are life for you. As it is written (Mishlei 4:22) "for they are life to him who finds them", and (Ibid. 3:8) "It is healing to your navel and marrow to your bones." R. Yitzchak says: If they have no illness, why do they need healing? But (the intent is:) "All of the illness which I placed upon Egypt, I will not place upon you" in this world; (and if I do place it upon you, it is as if I have not placed it upon you) "for I am the L rd who heals you" (in the world to come.) Shimon b. Azzai says: (If it is already written ) "heed," why is it (also) written "you shall heed"? I might think that if one wishes to heed, he is caused to heed later; to forget, he is caused to forget later; it is, therefore, written "heed, you shall heed," "forget, you shall forget." How so? If he wishes to heed, he is caused to heed at once; (if he wishes) to forget, he is caused to forget at once. He was wont to say: If a man wishes to heed, of his volition, he is caused to heed (even) against his volition. To forget, of his volition, he is caused to forget (even) against his volition. "Permission (to exercise one's free will) is given" — (Mishlei 3:34) "If (men would be) scoffers, He will (enable them to) scoff; and to (men who would be) humble, He will grant (them the) grace (to be so)." Others say (homiletically, on Exodus 22:25): "If chavol, tachbol" — If you cause (one) injury, you shall suffer (many) injuries.