מַתְקֵיף לַהּ רַב דִּימִי: וּמִמַּאי דְּהָכָא בְּחַמִּין שֶׁהוּחַמּוּ בְּשַׁבָּת פְּלִיגִי? דִילְמָא בְּחַמִּין שֶׁהוּחַמּוּ בְּעֶרֶב שַׁבָּת פְּלִיגִי?! אָמַר אַבָּיֵי: אֲנָא בְּעַאי דֶּאֱישַׁנֵּי לֵיהּ, וּקְדַם וְשַׁנִּי לֵיהּ רַב יוֹסֵף: מִפְּנֵי שֶׁסַּכָּנָה הוּא לוֹ.
Rav Dimi strongly objects to Rav Yosef’s point: And from where do you know that here it is with regard to hot water heated on Shabbat that they disagree? Perhaps it is with regard to hot water heated on Shabbat eve that they disagree. Abaye said: I wanted to answer him, but Rav Yosef answered him first: The dispute is certainly with regard to heating water on Shabbat, because failure to do so poses a danger for the baby.
כשם שמותר להגדיל את האש לצורך תאורה וחימום ביום טוב, כך מותר להגביר את האור בנורה חשמלית ואת החום בתנור חשמלי, וזאת בתנאי שהגברת האור והחימום נעשית על ידי הוספת זרם באותו חוט להט. אבל אסור להגביר את האור והחימום על ידי הדלקת חוט להט נוסף, מפני שזו הדלקה של אש חדשה.
Just as one may increase a flame to provide light or heat on Yom Tov, so too he may turn up the light of an incandescent bulb with a dimmer switch, or turn up the thermostat on an electric heater, on condition that turning up the light or heat increases the current to the same heating element. However, if it activates an additional filament, it is forbidden, as it is like lighting a new fire.
R. Benjamin Silber, Oz Nidberu, I, no, 34, notes that if Rashi's comment is to be understood in this manner, it would follow that, if use of solar heat in cooking should at any time come into vogue as a common practice, such cooking would have to be regarded as prohibited by biblical law. This, he argues, is already the case in Israel where solar heaters are commonly used for heating tap water. Accordingly, if Rashi's comment is to be understood in this manner, such heating constitutes a form of "cooking." R. Joshua Neuwirth, Shemirat Shabbat ke-Hilkhatah, 2nd edition (Jerusalem, 5739), chapter 1, note 127, presents a similar objection in the name of R. Shlomoh Zalman Auerbach. Magen Avraham, Oraḥ Hayyim 301:57, compares drying clothes in the sun to cooking by means of solar rays and rules that drying clothes in the sun on Shabbat similarly involves no biblical transgression. Rabbi Auerbach cogently notes that drying clothes in the sun is certainly a common and usual practice. If so, the clear implication of Magen Avraham's remark is that even conventional use of solar heat for cooking does not render the act biblically forbidden.
אָמַר רַב נַחְמָן: בְּחַמָּה — דְּכוּלֵּי עָלְמָא לָא פְּלִיגִי דִּשְׁרֵי. בְּתוֹלְדוֹת הָאוּר — כּוּלֵּי עָלְמָא לָא פְּלִיגִי דַּאֲסִיר. כִּי פְּלִיגִי בְּתוֹלְדוֹת הַחַמָּה: מָר סָבַר גָּזְרִינַן תּוֹלְדוֹת הַחַמָּה אַטּוּ תּוֹלְדוֹת הָאוּר, וּמָר סָבַר לָא גָּזְרִינַן.
Rav Naḥman said: With regard to heating food in the sun itself, everyone agrees that one is permitted to place food in the sun to heat it, as it is certainly neither fire nor a typical form of cooking. Likewise, with derivatives of fire, i.e., objects that were heated by fire, everyone agrees that it is prohibited to heat food with them, as heating with them is tantamount to heating with fire itself. Where they argue is with regard to heating with derivatives of the sun, i.e., objects heated with the heat of the sun. This Sage, who represents the opinion of the Rabbis, holds that we issue a decree prohibiting a person to heat with derivatives of the sun due to derivatives of fire, which are prohibited. People have no way of knowing how the cooking vessel was heated. If the Sages permit the use of objects heated in the sun, people will come to permit use of objects heated by fire as well. And this Sage, Rabbi Yosei, holds that we do not issue a decree. Even though it is prohibited to heat with derivatives of fire, heating with derivatives of the sun is permitted.
הַנּוֹתֵן בֵּיצָה בְּצַד הַמֵּיחַם בִּשְׁבִיל שֶׁתִּתְגַּלְגֵּל וְנִתְגַּלְגְּלָה חַיָּב. שֶׁהַמְבַשֵּׁל בְּתוֹלֶדֶת הָאוּר כִּמְבַשֵּׁל בָּאוּר עַצְמָהּ. וְכֵן הַמֵּדִיחַ בְּחַמִּין דָּג מָלִיחַ הַיָּשָׁן אוֹ קוּלְיָיס הָאִסְפָּנִין וְהוּא דָּג דַּק וְרַךְ בְּיוֹתֵר הֲרֵי זֶה חַיָּב. שֶׁהֲדָחָתָן בְּחַמִּין זֶה הוּא גְּמַר בִּשּׁוּלָן. וְכֵן כָּל כַּיּוֹצֵא בָּהֶן:
One who places an egg next to an urn so that it will roast slightly and it becomes slightly roasted is liable – since one who cooks with an effect of fire is like one who cooks with fire itself. Likewise one rinses an old salted fish or a Spanish kolias – which is a very small and delicate fish – in hot water is surely liable. For scalding in hot water is the completion of its cooking. And likewise anything that is similar to this.
אֲבוֹת מְלָאכוֹת אַרְבָּעִים חָסֵר אֶחָת. הַזּוֹרֵעַ. וְהַחוֹרֵשׁ. וְהַקּוֹצֵר. וְהַמְעַמֵּר. הַדָּשׁ. וְהַזּוֹרֶה. הַבּוֹרֵר. הַטּוֹחֵן. וְהַמְרַקֵּד. וְהַלָּשׁ. וְהָאוֹפֶה. הַגּוֹזֵז אֶת הַצֶּמֶר. הַמְלַבְּנוֹ. וְהַמְנַפְּצוֹ. וְהַצּוֹבְעוֹ. וְהַטּוֹוֶה. וְהַמֵּסֵךְ. וְהָעוֹשֶׂה שְׁנֵי בָתֵּי נִירִין. וְהָאוֹרֵג שְׁנֵי חוּטִין. וְהַפּוֹצֵעַ שְׁנֵי חוּטִין. הַקּוֹשֵׁר. וְהַמַּתִּיר. וְהַתּוֹפֵר שְׁתֵּי תְפִירוֹת. הַקּוֹרֵעַ עַל מְנָת לִתְפֹּר שְׁתֵּי תְפִירוֹת. הַצָּד צְבִי. הַשּׁוֹחֲטוֹ. וְהַמַּפְשִׁיטוֹ. הַמּוֹלְחוֹ, וְהַמְעַבֵּד אֶת עוֹרוֹ. וְהַמּוֹחֲקוֹ. וְהַמְחַתְּכוֹ. הַכּוֹתֵב שְׁתֵּי אוֹתִיּוֹת. וְהַמּוֹחֵק עַל מְנָת לִכְתֹּב שְׁתֵּי אוֹתִיּוֹת. הַבּוֹנֶה. וְהַסּוֹתֵר. הַמְכַבֶּה. וְהַמַּבְעִיר. הַמַּכֶּה בַפַּטִּישׁ. הַמּוֹצִיא מֵרְשׁוּת לִרְשׁוּת. הֲרֵי אֵלּוּ אֲבוֹת מְלָאכוֹת אַרְבָּעִים חָסֵר אֶחָת:
This fundamental mishna enumerates those who perform the primary categories of labor prohibited on Shabbat, which number forty-less-one. They are grouped in accordance with their function: One who sows, and one who plows, and one who reaps, and one who gathers sheaves into a pile, and one who threshes, removing the kernel from the husk, and one who winnows threshed grain in the wind, and one who selects the inedible waste from the edible, and one who grinds, and one who sifts the flour in a sieve, and one who kneads dough, and one who bakes. Additional primary categories of prohibited labor are the following: One who shears wool, and one who whitens it, and one who combs the fleece and straightens it, and one who dyes it, and one who spins the wool, and one who stretches the threads of the warp in the loom, and one who constructs two meshes, tying the threads of the warp to the base of the loom, and one who weaves two threads, and one who severs two threads for constructive purposes, and one who ties a knot, and one who unties a knot, and one who sews two stitches with a needle, as well as one who tears a fabric in order to sew two stitches. One who traps a deer, or any living creature, and one who slaughters it, and one who flays it, and one who salts its hide, a step in the tanning process, and one who tans its hide, and one who smooths it, removing hairs and veins, and one who cuts it into measured parts. One who writes two letters and one who erases in order to write two letters. One who builds a structure, and one who dismantles it, one who extinguishes a fire, and one who kindles a fire. One who strikes a blow with a hammer to complete the production process of a vessel (Rabbeinu Ḥananel), and one who carries out an object from domain to domain. All these are primary categories of labor, and they number forty-less-one.