Pesachim 74-76 Science,Torah and מחלוקת מציאות

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.


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.)


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.

Shabbos 77/ Parshas Bamidbar Machlokes in metzius and the size of an olive

It is a generally accepted rule in the world of “lomdos” (analytical learning)  that even where a dispute amongst Chazal appears to be about facts [can a child father a child or not ,is one example of many (Sanhedrin 69a) as is דם מפקיד פקיד או מחבר מחבר ( Kesubos 5b) ] , if it is something that can be readily checked, we attempt to show that the argument is really based on something other than just superficial facts ( is someone who has matured to the point that he can father a child still considered a child, or is דם perceived halachically as if it is מפקיד פקיד or מחובר מיחבר , for possible, if not problematic examples )

When the dispute is about facts that cannot be easily verified by observation , like some historic arguments ( like how old biblical figures were at certain times ) or unobservable phenomena , there is no need to do this, as the dispute can simply be explained as being based on different interpretations of the evidence or pessukim.

On this daf, we learn the chiddush of Rabbi Nosson that whilst the minimum quantity of wine required to transgress the melacha of carrying is a reviis, if the wine is conjealed to the point that it is more solid than liquid, the minimum measure of a kezayis  applies instead. 

The Gemora understands that this is because when a reviis of wine congeals, it shrinks to a kezayis.

Rav Yoseif then suggests that this view is the same as that expressed by Rabbi Yossi son of Rabbi Yehuda regarding the blood of an animal that dies without halachik slaughter (a neveila)

A kezayis of flesh from a neveila can make someone impure, but what about liquid blood?

Beis Shamai holds that blood of a neveila is considered like the flesh and does not cause impurity, whereas Beis Hillel holds that it does.

Rabbi Yossi holds that even Beit Hillel require at least a reviis of blood to make someone impure, because if a reviis of blood were to congeal, it would shrink to a kezayis.

Rav Yoseif understands from this that both Rabbi Nosson and Rabbi Yossi agree that a reviis of liquid is the equivalent of a kezayis of solids.

However- Abaya questions this assumption, and suggests that on the contrary, they may in fact totally disagree.

He points out something that students of physics should recognize, namely the fact that not ALL liquids are equal (nor are all solids.)

Blood as a liquid is much thicker than wine as a liquid, and therefore it does not have to congeal as much to become solid.

It therefore follows that if a reviis of wine is required to produce a kezayis of congealed wine, less than a reviis of blood is required to produce a kezayis of congealed blood.

and if a reviis of blood is indeed required to produce a kezayis of congealed blood, then MORE than a reviis of wine is required to produce a kezayis of congealed wine.

If so, it follows that the amount of liquid blood and wine respectively that are required to produce a kezayis of the congealed equivalent is actually a matter of dispute between Rabbi Nosson and Rabbi Yossi!

This appears to be a classic example of a machlokes metzius (factual dispute) that should be easily checked out by experimentation, rather than debated.

We also need to explain what Rav Yoseif himself was thinking- surely he was not unaware of the basic fact that blood is thicker than wine?

I think that one can perhaps explain Rav Yoseif’s view by way of the important principle that Halachic rules are very often NOT based on precise facts, measurements, or statistics, but rather on fixed approximations based on perceptions.

When it comes to working out the required dimensions of a round Sukkah (Sukkah  7b) or of a round beam on top of an eruv (Eiruvin 13b), the Mishna simply applies a round ratio of 3 to 1 between the circumference of a circle and its diameter, substantially less than even a reasonable estimation of pi, which at the time of Chazal was historically already known to about 2 decimal places. (see also Bava Basra 14b)

Whereas Tosfos in Eruvin are bothered by this discrepancy, and seem to understand that the Mishna and certainly the accompanying sugya are taking the ratio of 3 to be precise, the Tosfos haRosh there actually suggests that Chazal are intentionally imprecise, based on precedent from the passuk describing the pool that Shlomo built.

We see from there that in at least some areas of halacha, the Torah does not require precise measures that fit absolute scientific reality, but rather simply estimations that Chazal taught us, which are perceptibly close to the real measurement but simple enough to apply across the board.

This principle is actually applied  by the Chazon Ish in very novel ways to issues relating to the dateline (subject of 2 previous posts) , whereby he allows it to follow the coastline and deviate somewhat from the 90 degrees he claims is its absolute geographic position.

Perhaps, Rav Yoseif believes that the relationship between solid and liquid measurements is also an example of a case where the Torah allows for an estimation, and we are not required to make allowances for the precise density of the liquid or the solid.

Similarly, perhaps Abaya holds that either there is simply no justification for applying such estimations in this case, or that the variance in density and resulting solid size is too high to be comparable to the pi analogy.

Perhaps a similar idea can be used to explain the machlokes that Abaya sees in Rabbi Nathan and Rabbi Yossi’s rulings, in that they are not debating the facts- both agree that neither a reviis of wine or blood is the precise equivalent of a kezayis of their congealed version.

Or perhaps the reviis to kezayis ratio is only true of some hypothetical congealable liquid with a density somewhere between that of wine and blood, and the dispute is on which side of the line to estimate?

One thing we seem to see  for sure from this daf at least, is that there is a direct relationship between the side of a kezayis and  that of a reviis, and if one follows a certain view as to the size of the kezayis, one should certainly follow the same view in regards to the size of a reviis!

in this parsha, we are told that 20 is the determining age for maturity regarding being counted in a census, going to the army, etc.

We also know from numerous sources (among them a few daf ahead in our Masechta), that it is also the also the determining age for liability to punishment in the hands of heaven.

We know from the Torah sheBaal Peh that when it comes to many other matters, including the obligation to perform mitzvos and liability to punishment in Beis Din, the cut-off age is 13.

Although it is clear and obvious that there is a huge variation in maturity amongst different 20 year olds, and different 13 years olds, we see here clearly that at least to some extent, halacha is not determined by precise objective factors, but by fixed estimations- the same argument can be applied to most halachik measurements.

However, unlike what we saw regarding pi and perhaps the other examples we mentioned, the actual age or measurement provided by the Torah here appears to be absolute- a minor who is one minute younger than 13 will be exempt , and one 13 or older will be liable.

A young man 1 day younger than 20 will be exempt from the army, and one day later will be recruited.

I suggest that wherever it is simply and straightforward to make a fixed precise measure, this is indeed what the Torah does, but when it is impossible (like in the case of pi) or extremely difficult for people to use  a precise fixed measure, an estimated fixed measure is provided instead.

so much to say still, but shabbos beckons- please share your thoughts either way.

Shabbat Shalom and Yom Yerushalayim Sameach