Bishul 10

Rav Yitzḥak bar Avdimi said: One time I followed Rabbi Yehuda HaNasi into the bathhouse on Shabbat to assist him, and I sought to place a jar of oil in the bathtub for him, to heat the oil somewhat before rubbing it on him. And he said to me: Take water from the bath in a secondary vessel and place the oil into it. The Gemara remarks: Learn from this comment of Rabbi Yehuda HaNasi three halakhot: Learn from it that oil is subject to the prohibition of cooking. This explains why he prohibited placing it in the bathtub. And learn from it that a secondary vessel is not hot and does not cook. And learn from it with regard to oil that warming it is tantamount to cooking it.

And we learn from this that a Kli Sheini doesn't cook - (Question) Why is a kli sheini different than a kli rishon? If it's yad soledes then even a kli sheini should cook, and if it is not yad soledes then even a kli rishon won't cook? (Answer) Since the kli rishon was on the fire it's walls are hot and it will retain the heat of it's contents for a long time, therefore it was given a measurement: if the contents are yad soledes then it is forbidden. While a kli sheini even if it's contents are yad soledes it is permitted [to put something that would potentially cook inside it] since the walls are cold and they will cause the contents to continuously cool down.

...[Following a discussion about the status of a kli rishon with regards to cooking, the gemara continues:] Everyone admits that a kli sheini is permitted [not an issue of cooking]. What is the difference between a kli rishon and a kli sheini? (Answer 1) Says Rabi Yosi, here [kli sheini]

the hand can touch [meaning it is not so hot that one's hand will recoil from it], while here [kli rishon] the hand cannot touch. (Answer 2) Says Rabi Yonah, in both cases it is (yad soledes) too hot to touch, rather the difference is that they made a preventative measure by kli rishon [so that one wouldn't end up putting something into hot water that was actually on the fire] and they didn't by kli sheini (even when it is yad soledes). Said Rabbi Mana this bowl of rice is a proof to my father (Rabi Yonah). This bowl of groats is a proof to my father, for it is poured from one place to another [from a kli rishon to a kli shenini] and yet it remains hot! (So we see that there is no temperature [physical] difference between kli rishon and kli sheini, rather the difference is simply where they forbid it [a halachic difference]).

...in truth in the Yerushalmi it says that they made a preventative measure by kli rishon even if it is not yad soledes bo. The Tashbetz writes this as well. See in Yoreh Deah Siman 94 in the Taz in the name of Rashal [Rav Shlomo Luria].

...any "heat" is specifically [at least] yad soledes, though if it is not that hot even if it is on the fire it is not forbidden. This is said by the Rishonim and Acharonim in many places and the Maharshal writes this as well. Except that afterwards, he follows this by saying that he would be scared to be lenient if a hot kli rishon is on the fire even if it is not yad soledes because we find that the Yerushalmi holds that there was a rabbinic decree forbidding such a case. [Questions the Shach:] I don't understand him? In the Yerushalmi it is clear that when it is yad soledes, then it is forbidden, but if one can touch it then it is permitted according to all opinions [so where does he see that the Yerushalmi forbid any case where it wasn't yad soledes?]...

The whole time that it is yad soledes - for if it is not yad soledes there is no issue of cooking like the gemara writes in [Shabbos] 40b. See Tosafos there. And that which we write in this whole siman that it has to be yad soledes because if it is not yad soledes then even in an oven it is permitted, as we wrote in siman 253 and the Yerushalmi says this as well in the beginning of the 3rd perek of Shabbos while discussing ovens, see there. And that which the Magen Avraham writes in seif katan 28 in the name of the Yerushalmi the opposite, he was not accurate for it says there [Yerushalmi Shabbos] Perek 3 at the end of the Second Halacha: Rabbi Zeirah said in the name of Rabbi Yehuda one may warm something up in a place where one's hand can go [meaning it is not yad soledes] though it is forbidden to warm something up in a place where one's hand cannot go [because it is yad soledes there]. What is the limit? The limit is where he sticks his hand there and it gets burned. Everyone agrees that a kli sheini is permitted. What is the difference between a kli rishon and a kli sheini? Says Rabbi Yosi here the hand can touch and here it cannot. Says Rabbi Yonah, in both cases the hand cannot touch, rather they made a preventative measure by kli rishon and not by kli sheini. Meaning to say that even though by a kli rishon which one only intended to warm up, they made a prohibition to do so for fear that one would leave it there until it did heat up to yad soledes bo like the Rosh writes in 3:10 and the Ran. The Yerushalmi calls "the hand cannot touch there" what the Bavli calls "the hand recoils from it".