Exode 15:26

and said, If you will truly hearken to the Word of the Lord your God, and do that which is right before Him, and will listen to His precepts and keep all His statutes, all those evil things that I laid upon the Mizraee I will not lay upon thee: but if thou wilt transgress against the word of the law, upon thee shall they be sent. If thou convert, I will remove them from thee; for I am the Lord thy Healer.

IF ‘SHAMO’A TISHMA’ TO THE VOICE OF THE ETERNAL THY G-D. Rabbi Abraham ibn Ezra explained that “[shamo’a tishma here] means ‘to understand’ would mean “surely listen” or “diligently hearken,” but wherever this expression is followed by the letter lamed — as in this case, [shamo’a tishma l’kol] — or a beth, it means ‘understanding.’ This is why Ibn Ezra explained it here to mean “if you will have a full understanding of His commandments.” the purport of that which He has commanded you to do. And thou wilt do that which is right in His eyes — this implies the positive commandments — and wilt give ear to His commandments — this implies the negative commandments.” [Thus the language of Ibn Ezra.] And in the Mechilta, the Rabbis have said: “And thou wilt do that which is right in His eyes, this means in business dealings. This teaches us that if a person is honest in his business dealings, and the spirit of his fellow creatures finds pleasure in him, it is accounted to him as though he had fulfilled the entire Torah.”. The sense thereof is thus: “If you will do that which is right in G-d’s eyes, meaning if you will be honest in your business dealings, it will be accounted to you as though you had given ear, etc.” I will further explain this when I come to the verse ‘And thou shalt do that which is upright and good,’ if the good G-d will show me goodness.

(1) ANY OF THE DISEASES THAT I BROUGHT UPON THE EGYPTIANS. That I turned their waters into blood (Ps. 105:29), and they did not have water to drink.

(2) I WILL NOT BRING UPON YOU, FOR I THE LORD AM YOUR HEALER. In that I have "healed" the waters, as this expression is written with respect to Elisha, who "healed" the water. (II Kings 2:21)

(3) ANY OF THE DISEASES. It refers to [those of] water, as it is written (Ex. 23:25), "He will bless your bread and your water. And I will remove sickness from your midst."

אם שמוע תשמע לקול ה' אלוקיך, to accept the statutes which would follow as a way of life, not merely as emergency regulation in the desert, and if from now on they would הישר בעיניו תעשה והאזנת למצותיו, וגו, carefully observe G’d’s commandments endeavouring to live according to their understanding of what is pleasing to Him, then they would be permanently spared any of the afflictions that G’d had brought upon the Egyptians. The implication was that betraying G’d’s trust would have painful consequences. The entire experience is a reminder of the manner in which the Rabbis relate to a potential convert to Judaism. One does not present Judaism in rosy attractive colours so as to encourage him to convert, but on the contrary, one apprises him first and foremost of the עול המצות, “the yoke of the commandments,” telling him of all the things he would have to forego by joining the Jewish people, i.e. the Jewish religion. Moreover, one advises him that whereas up until now he was able to eat certain foods, such as chelev, not only would he no longer be able to eat these foods, but violation of the commandment would carry the penalty of karet, loss of entitlement to his afterlife, etc. Only after he has duly absorbed this information does one proceed with teaching him the intricacies of Judaism. What happened at Marah was preparatory to accepting Judaism at Mount Sinai. If the people had not gone through this process it is doubtful that they would have said נעשה ונשמע, “we will do, now let us learn G’d’s Torah,” when the time came for this a few weeks later.

כי אני, ה' רופאך, “for I, the Lord, am your physician.” The Torah here speaks of preventive medicine. It tells us that G–d, i.e. His Torah, teaches us how to forestall sickness. Physicians are in the habit of telling their patients what to eat and what not to eat so that that they will not become victims of certain common diseases. This is why the verse commences with the warning to pay heed to what your Physician tells you before you become a victim to these diseases. We are to consider the performance of the Torah’s commandments as preventive medicine. Our author also quotes Isaiah 17,11 as support for his interpretation. The prophet wrote: נד קציר ביום נחלה וכאב אנוש, “on the day that you plant you see it grow.” He also quotes proverbs 4,22, where the author concludes his advice in verse 20 by stating: ולכל בשרו מרפא, “and healing for all his flesh.” (Attributed to Rabbi Joseph Kara). The words: כל המחלה in our verse above is interpreted as having the same meaning as the same word in Samuel II 12,2: ולרש אין כל, “and the poor man does not have anything,” and as in Psalms 78,38: ולא יעיר כל חמתו, “and He did not give full vent to His fury,” i.e. “any of the different kinds of disease I will not inflict upon you.”