A major theme of the beginning of פרק כיצד צולין  is the requirement that the קרבן פסח  be                  “צלי אש ולא צלי מחמת דבר אחר” (roasted by fire and not by anything else.)

This is derived from the double mention of the word “צלי אש”  in the פסוקים  (Shmos 12), which teaches us that not only must the קרבן פסח  be roasted by direct flame and not the heat of the vessel or liquid , but even things that might be considered the same as fire for other things requiring a fire  (such as a מכוה -leprous burn-which may come even from burning metal), are not considered as fire for the קרבן פסח , except for an actual flame itself.

As such, the פסח  may not be roasted with a metal spit, or through direct contact with the walls or floor of the oven, as part of the roasting process would then be performed by the heat of the metal spit or of the oven surface, and not directly by the fire.

There is much discussion regarding  the scope of these rules, during which some essential principles of אסור והתיר (contact between forbidden and permitted foods) is derived.

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One of the most important rules pertains to how permitted items that have absorbed the taste of forbidden items may be freed of their forbidden status, namely the rule of “כבולעו כך פולטו”- in the same way that the forbidden taste is absorbed, so it is expelled.

A common application of this rule is that vessels that were used for roasting non-kosher food over a flame without liquid require לבון (direct, dry heat of a flame in order to be koshered.)

In contrast, vessels that were exposed only to hot liquids or foods cooked in liquids may generally be koshered by הדחה  (immersion in boiling water.)

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Two other important rules are subject to debate.

1. חם מקצתו חם כולו- when it comes to metal items (which conduct heat), if part of the item is hot, the entire item is viewed as hot. This is the view of the Mishna on daf 74a which forbids using a metal spit for roasting the קרבן פסח , assuming that the part of the metal spit inside the animal though less exposed to the fire is heated by the part outside it , something disputed by Rabbi Yehuda in a Beraisa brought by the Gemara on daf 74a.
2. תתאה גבר – When a hot kosher or non-kosher item makes contact with a cold item of the opposite halachic status , do we view this stringently as hot contact, or leniently as cold contact?

It depends on whether the hot item is on top or on the bottom, but the rule is still subject to debate. רב  rules that עילאה גבר  – the item on top prevails. This means that if the item on top is hot, the contact is treated as hot contact, whereas if the item on top is cold, it is treated as cold contact.

In contrast, שמואל  holds that  תתאה גבר, the bottom item prevails- if the bottom item is hot, is is treated as hot contact whereas if the bottom item is cold, it is treated as cold contact.

The Gemara on daf 76a brings various proofs from the Mishna on daf 75b for the view of רב but they are all rejected. It then brings 2 proofs from ברייתות in support of Shmuel, seemingly given him the last word (see Rashi ד”ה “ושמואל who indeed rules this way), but clarifying that even if the cold item is on the bottom item, it is not treated exactly the same as cold contact but rather more leniently that hot contact- the kosher item is not rendered non-kosher in its entirely but the section of it that came in direct touch with the non-kosher item on top requires קליפה (peeling off)- see Tosfos ד”ה “תניא”  who discusses the situation where the kosher item is liquid and cannot be “peeled.”)

Both the above disputes share the common property that they appear at face value to be based on מציאות (factual matters) that can be easily tested.

The question of whether part of a metal spit being hot causes the rest of it to become hot is a scientific question easily answered by experimentation, as is the question of whether the top or bottom item being hot causes the two items to absorb taste from one another.

In truth, the scientific observation that hot air rises seems to indicate that if the hot item is on the bottom, it imbibes taste into the upper cold item but not the other way round, regardless of whether the permitted item is on the bottom or top, something which neither רב  nor שומאל  seem to acknowledge.

For example, if a cold kosher item is on top of a hot non-kosher item, then the taste of the hot non-kosher item should rise and be absorbed into the cold kosher item on top, as שמואל  indeed holds.

However, if a cold non-kosher item is on top of a hot kosher item, then from a scientific point of view, it seems that there is no way for the cold non-kosher item’s taste to rise and be absorbed into the hot kosher item below and besides for the area of direct contact which might require קליפה, there seems to be no reason to forbid the upper cold kosher item- yet שמואל  would indeed forbid it in this case too, seeing as the important factor to him is whether the hot item is on the bottom or not, not whether it is the kosher item or the non-kosher one!

In the world of  למדנות (lomdus or analytical learning), we generally try to avoid interpreting such disputes superficially and search for a more conceptual basis to the argument.

Additionally, although there might be some debate amongst the ראשונים  regarding how to relate to statements of Chazal that appear to be based on the possibly faulty science of their time, these both seem to be easily observable rules which are not dependant on relatively modern scientific research!

Perhaps one can suggest that the first dispute does not resolve around whether the part of the metal spit in direct contact with the animal is heated by the part of it that is exposed to the flame, but whether the degree of heating is considered sufficient for us to consider the animal as being roasted partly by the spit and not roasted in its entirety directly by the fire as required.

We see what might be a similar  distinction when it comes to the distinction of cooking in a כלי ראשון  on Shabbos  as opposed to cooking in a כלי שני.

Although the contents of the כלי שני  could be just as hot as the contents of a כלי ראשון , certainly to the point of יד סולדת בו (the min temperature that water needs to be in order for one to quickly withdraw one’s hand from it after testing it, which serves as the minimum temperature required for cooking on shabbos.), the heat of the כלי שני  is considered secondary, being derived from the contents of the כלי שני  and thus has less power to effect the cooking process.

Similarly, Rabbi Yehuda could opine that even though the entire rod is hot, the unexposed part of the rod has gained its heat from a secondary source, namely the exposed part, and this lacks the energy to effect significant halachik cooking, thus not invalidating the direct flame-roasting of the קרבן.

In contrast, the mainstream view rejects this parallel because unlike the כלי שני  whose contents might be just as hot but which itself lacks heat and thus absorbs the heating energy of the contents sparing  anything else placed inside it from the halachik cooking process, the unexposed part of the spit is constantly absorbing new heat from the exposed part, and thus far more capable of effecting halachik cooking to the קרבן, thus invalidating the direct flame roasting.

Even if this analysis passes scientific rigor, or if we accept that halachik definitions are not always the same as scientific, it seems harder to take such an approach in the second dispute.

Here the question is clearly not whether the type of heating caused by  a fixed degree of heat is halachically considered cooking or not, it is about whether the bottom item or the higher item is significant in determining the results of the contact.  This seems to be a מחלוקת מציאות by definition!

While searching the Responsa Project for Achronim who might discuss this issue, I came across a פרוש  on Pesachim by Rav Shlomo Ganzfried (פני שמואל פסחים עו.) , the famous author of קצור שולחן ערוך , who points to a תשובה (responsa) of the נודע ביהודה (קמא יו”ד כח) who asks this precise question.

While pointing out that there are many similar debates in matters of אסור והתיר  that appear to be מחלוקת מציאות that can be easily resolved by experiment, he notes that these debates where never resolved that way.

For example, אhere is famous debate (Chullin 98a) whether forbidden foods are בטל בשישים  (nullified) by 60 times the quantity of permitted foods) or only בטל במאה (nullified by 100 times the quantity.)

This too seems to be a מחלוקת מציאות  as the main issue is whether the אסור  imparts of its taste to the התיר  when the היתר  is more than 60 but less than 100 times the אסור.

The same question can be asked regarding the debate whether מליח כרותח  (salting is like boiling.)

He presents the thesis that the rules of when taste is transferred are not based on objectively proven criteria, as just because an item is able to impart taste, it does not always do so.

Chazal set certain thresh-holds based on what appeared logical to them,  that determine when we are חושש for this happening, but that does not mean that every time there is less than 60 times the אסור  taste is always imparted or vice versa.

As such, ascertaining these things is hard enough to do via experimentation that debates around them are a justified form of מחלוקת מציאות .

How the availability of modern statistical sampling methods might improve our ability not only to experiment but to assess how significant the results of the experiments are is not addressed in the נודע      ביהודה, and opens questions which are beyond the scope of this post.

These posts are intended to raise issues and stimulate further research and discussion on contemporary topics related to the daf. They are not intended as psak halacha.