Daf Yomi recently learned the gemara in Shabbos(40b) which states a principle that כלי שני אינו מבשל. The question is why not?
There are 2 possible approaches:
1. It is a דין. In other words the halacha states that cooking in a כלי שני is not called cooking. We find for example that בישול בחמה is permitted.
2. It is a מציאות, the gemara is telling us an empirical fact, that in general a כלי שני doesn't cook.
Many of the Rishonim state explicitly that כלי שני אינו מבשל is a מציאות. The Rashba there in Shabbos, the יראים quoted by many other rishonim and others. Tosafos there also seems to say this because they explain the difference between a כלי ראשון and a כלי שני based on מציאות, that a כלי ראשון has hot walls while a כלי שני doesn't.
The gemara about קלי הבישול seems to support this position. The simple reading of the gemara (39a and other places) is that there are certain things that are easily cooked and you are חייב if you cook them in a כלי שני. If it was a din then how wcould you be chayav by קלי הבישול?
None of the Rishonim (that I saw) say explicitly that it is a דין (however see Tosafos shabbos 42a where they discuss a hot bath and other Rishonim (Ramban, Ritva) which could be interpreted this way). However the אור שמח (hilchos shabbos perek 9) goes with this idea explicitly, and explains it as follows. He says that the gemara states that cooking in halacha is defined as by fire or the result of a fire. He says that a כלי שני is so far removed from the fire that it can't be called תולדות האש and therefore is not considered cooking in halacha. This is similar to the din that cooking in the sun is not considered cooking. He explains that קלי הבישול is a gezera because people do cook קלי הבישול in a כלי שני therefore they prohibited it m'drabbanan.
There are a number of practical differences in halacha regarding this question, I will mention 2 of them:
1. If the כלי שני is really hot. The Chayei Adam based on a Rambam in Maaser Sheni holds that if the כלי שני is boiling hot (if you touch it you will get burned) then the rule of כלי שני אינו מבשל doesn't apply. This is clearly going with the 2, that it is a מציאות, according to the אור שמח it shouldn't matter.
2. Is there a קולא of כלי שלישי? The Mishna Berura quotes a Pri megadim who is מיקל by a כלי שלישי by קלי הבישול, that you would be permitted to put them in a כלי שלישי. The Chazon Ish and others disagree. they hold that there is no difference between a כלי שני and a כלי שלישי based on Tosafos, both have cold walls and both have the same amount of heat. If you hold like 2 then the kula of כלי שלישי makes no sense. However according to the אור שמח that כלי שני אינו מבשל is a din and the humra by קלי הבישול is only a din drabbanan, then it makes sense to say that the gezera was only made on a כלי שני and not on a כלי שלישי.
Interestingly enough, the Mishna Berura quotes both of these dinim. He is machmir like the Chayei Adam which is based on מציאות and מיקל like the Pri Megadim which seems to be based on the fact that it is a din and not a מציאות.
At first glance opinion 1 (din) seems much more logical then 2. It seems very difficult to say that the gemara is telling us a מציאות without qualifying it. If the Chayei Adam is right how come the gemara didn't warn us about it. The Chayei Adam's scenario is not so uncommon and leads to an issur d'oraysa. The gemara's statement lends itself to be interpreted as a general principle in halacha not a מציאות. Therefore I was very happy to see the אור שמח expound the position of din explicitly.
There is another way to understand Tosafos in Shabbos. Tosafos holds that כלי שני אינו מבשל is because it is not the normal way to cook in a כלי שני and tosafos is just explaining why, because the walls are cold and therefore people don't generally cook in such a כלי. This is very similar to the אור שמח that really it is a din not a מציאות. The difference would be if you had a כלי שני that people did cook in, or food that people normally cook in a כלי שני such as קלי הבישול then it would be an issur d'oraysa according to Tosafos while according to the אור שמח it is never an issur doraysa.