Regarding the idea that Jewish law is macroscopic.
I remember from the period when I was forced to go through a private Arachim seminar (which didn’t succeed), a bitter argument about killing a louse on the Sabbath.
I argued that in light of modern research, one should be stringent regarding a Torah prohibition (were it not for the fact that it is an unneeded labor) and not kill lice on the Sabbath.
For tactical reasons, that rabbi from Arachim insisted on broadly permitting this act. He argued that since the louse’s “reproduction” is not visible to the human eye, it is considered as though the louse does not reproduce.
I did not accept his excuse; it felt ad hoc to me.
And regarding his actual claim, I told him that I see a major difference between the cases in which the rule that Jewish law is not microscopic is applied, and our case.

When we discuss whether there is a prohibition against eating bacteria, we can say that since they are not visible, there is no halakhic reference to them. The Torah did not prohibit what cannot be seen with the naked eye.
But in our case, the louse is visible to the eye. What is not visible is the process of reproduction, which is a condition for something’s being a “living creature.” Something that does not reproduce is not defined as a living creature.
Once we have already related to the louse because we saw it with our eyes, it is harder to determine that it is not a living creature just because the reproductive process cannot be seen with the eyes. On the contrary, let us bring a microscope and see.

(By the way, the question is of course not practical…)
What does the Rabbi think?

Aharon, it is not true that something which does not reproduce is not defined as a living creature (otherwise it would be permitted to eat a louse).
What is true is that the labor of taking a life is prohibited only for “significant” living things, whereas something that does not reproduce but is spontaneously generated from sweat has a less significant form of life.
The reasoning that a louse is considered, even nowadays, as generated from sweat because its reproduction is microscopic was not invented by that Arachim lecturer, but by great authorities (such as Rabbi Shlomo Zalman Auerbach and others), and what lies behind it is that as long as it is not evident to the naked eye that this creature reproduces, its life is not considered “significant life.”
There were of course others who disagreed, and there is definitely logic to that. Especially if one takes into account that lice eggs actually are visible to the eye, except that they can be confused with flakes of scalp. And in the Talmud, as is known, it is mentioned that the Holy One “feeds the world from lice eggs to the horns of wild oxen” (the Babylonian Talmud was forced to reinterpret the expression as meaning a type of lice and not literally. But according to the Jerusalem Talmud, where the permission to kill a louse according to Beit Hillel stems from its short lifespan and not because it does not reproduce, the phrase “lice eggs” can be understood literally).

Aharon, I completely agree, but there is no need to get to your distinction.
The principle that Jewish law is not microscopic is sometimes used for apologetics, and there it should be rejected. But it is true that Jewish law deals with things as they appear and not with microscopy. In the case of the louse, apologetics has no place because it is impossible and unnecessary to read that into the words of the Sages, regardless of your distinction.
After all, the Sages said that it does not reproduce (and certainly if they said that it is generated from sweat). They did not say that it really is generated through reproduction but that this just cannot be seen. So it is clear that they were factually mistaken, and these excuses do not fit their words. This apologetics is nonsense.
Nowadays people are comfortable leaving the Jewish law as it is and using microscopy as apologetics. But there is no point in that, because there is no reason to make apologetics for a mistake. Granted, for something written in the Torah it is not reasonable that there would be an error on God’s part, and there maybe there is room for apologetics. But with the Sages there are mistakes, and there is no reason there should not be, so there is no need and it is not right to save them with forced interpretations. And the claim about “significant life” also falls into this category (how many of us have seen the reproduction of flies, or even lions?).

Why did you decide that in the case of the louse this is apologetics?
After all, the logic behind it is that it is unreasonable that Jewish law, which is meant to be a way of life, should require laboratory tests, something entrusted to experts (and even among experts this exists only in recent generations). And to the eye, a louse does not appear to reproduce.
This is not apologetics (or at least not necessarily apologetics. For even someone who accepts the obvious fact to anyone with eyes in his head, that the Sages erred in scientific facts, can adopt this).

Regarding a shaving machine—there specifically the microscopic argument is weaker, because with the eye one cannot see whether one blade cuts or two, so why choose דווקא the first possibility? And more than that, here we are dealing with a mechanism engineered by human beings, so reason suggests that you should turn to engineers on the question.

To Rabbi Michi:

Let me sharpen the point again; I think maybe NM also meant this.

I agree that many times there is room for the principle that Jewish law is not microscopic, and it is not always apologetics.

In our case, it makes no difference whether the Sages meant to say that the louse does not reproduce, or only that it is not visible. The question is whether, in light of the scientific findings, the Jewish law should be updated.
Even if the Sages literally meant that the louse does not reproduce, and we determine that it does, there is still room to continue with their halakhic ruling.
Should a visible creature, whose reproductive process is not visible, be considered worthy of being classified as a living creature?

And to NM:
I know that this reasoning was not invented by that lecturer, and it is commonly quoted in the name of Rabbi Shlomo Zalman Auerbach. Here are the details:
http://www.havabooks.co.il/article_ID.asp?id=1689

What bothered me was that lecturer’s tactic. If he had looked, he would have found the words of Rabbi Yosef Shalom Elyashiv (see Orchot Shabbat), who tends to forbid killing lice on the Sabbath (following the reasoning of Pachad Yitzchak). That person was not seeking the truth. He defined the goal in advance and then shot the arrows.
He would not in any way, not in this discussion and not in others, admit that the Sages might have erred in facts (because if he admits it on this issue, afterward I will continue to weightier topics, like a woman’s testimony). He stood like flint and explained away every error apologetically.
Even when I argued that there is a difference between a creature that is itself not visible and one that is visible but only its reproductive process is not visible, he “really did not understand what I was saying.”
In short, the argument was not about the specific issue. Even though I do not agree with Rabbi Shlomo Zalman Auerbach’s reasoning, I do not think it is absurd. The argument was about methodology. In my eyes, he was not an honest person, but an apologist.

The whole ideological tension around the matter of killing a louse on the Sabbath is ridiculous. There are people who prefer to permit desecrating the Sabbath rather than blemish the (new) faith that the Sages were outstanding doctors, scientists, and astronomers, and that everything they said on these matters is eternally valid.
This is a purely halakhic question. And there are many like it in all areas of Jewish law. I completely understand (and somewhat support) the approach that forbids changes, at least as long as they will never enter the consensus and it would become like two Torahs, Heaven forbid. But one must not mix considerations of faith and worldview into it.
And as for the Arachim lecturers, they should ask themselves whether they are not entering the category of “Cursed is he who does the work of the Lord deceitfully.”

It wasn’t “the work of the Lord” — because he demanded (and also received) payment.

To Rabbi Michi (I’m repeating myself, because you didn’t answer me).

Let me sharpen the point again (because I didn’t quite understand your words, and I think maybe NM also meant this).

I agree that many times there is room for the principle that Jewish law is not microscopic, and it is not always apologetics.

In our case, it makes no difference whether the Sages meant to say that the louse does not reproduce or only that the matter is not visible. The question is whether, in light of the scientific findings, the Jewish law should be updated.

Even if the Sages literally meant that the louse does not reproduce, and we determine that it does, there is still room to continue with their halakhic ruling (for a different reason: that the process is not visible).

So is a visible creature, whose reproductive process is not visible, fit to be considered a living creature?

I don’t understand the source for such a reasoning. If the Sages were mistaken about reality and established Jewish law on that basis, what is the point of maintaining the Jewish law on the basis of other reasonings?
That reasoning has to be discussed separately. And in my opinion, from the fact that the Talmud needed to arrive at the point that they do not reproduce, it is proven that if they did, then it would be forbidden.
And to NM, with a shaving machine the engineers are not relevant simply because they created the machine; rather, once the machine is before us and we see with our own eyes a gap between the blade and the skin, we have to determine whether on that basis to permit it or based on laboratory tests. But the fact that they created the machine does not give them authority.

Aharon,
I answered, and for some reason my reply disappeared. But now I see that Yisrael answered you in my place (more power to him).

I don’t understand your answer.
The reason the Talmud used the claim that lice do not reproduce, and did not use Rabbi Shlomo Zalman’s reasoning, is that it really believed that they do not reproduce, and therefore it used the stronger claim.
Had it known that they do reproduce, it might have used Rabbi Shlomo Zalman’s reason.

The question is whether Rabbi Shlomo Zalman’s claim makes sense in and of itself.
There is a rule that a living creature (with respect to killing it on the Sabbath) is only one that reproduces.
There is a rule that Jewish law does not relate to a reality that is not visible to the naked eye.
The question is whether you do 1+1, and if so then once again we return to the point that lice do not reproduce.
Or,
that the second rule has exceptions (as in the distinction I wrote above).

We answered that. The answer is that had the Sages not said this, and had there not been motivation not to change the Jewish law, Rabbi Shlomo Zalman would never have thought to say such a reasoning. Would he have independently innovated a rule permitting violation of the Sabbath in a Torah-level prohibited labor by force of such a reasoning?
You were also answered that the Sages themselves did not agree with this reasoning. After all, if they had accepted it, they would not have needed to resort to the claim that a louse does not reproduce. According to your view, even if it does reproduce, it is permitted to kill it, because its reproduction is microscopic.
And since there is no need to defend the rabbinic halakhah, which is mistaken from the outset, with new reasonings, there is no point in discussing them. Note that by force of this reasoning they permit Torah-level Sabbath violation. Who would ever think of doing that based on reasoning alone? Just because we imagine an analogy, should we act on it?!
As for the reasoning itself, I already answered that too. Every child today knows that lice come from eggs, so it is not true that this is something microscopic that is unknown to us. It is known to us much more than how lions, bees, or cats reproduce, which none of us has ever seen.

Yes and no.
Had the Sages not said that a creature that does not reproduce is not included in the forbidden Sabbath labor, then indeed Rabbi Shlomo Zalman would not have innovated this on his own. Because the criterion of reproduction as a necessary condition for Sabbath labor is not trivial.
But once they innovated the law regarding a creature that does not reproduce, there is reasoning to say that any creature whose reproduction cannot be seen with the eye will not be considered as reproducing.
It is entirely possible that Rabbi Shlomo Zalman and those with him would have said this even about a creature that is not a louse, but which appears to the eye to be generated from human bodily secretions (for example, intestinal worms commonly found in children), even though only the louse appears in the words of the Sages. Therefore I would not treat this as apologetics.
In other words, I am not arguing that your reasoning is not strong, and that practically it is hard to permit; I am arguing only that those who hold otherwise are not necessarily doing so in order to defend the Sages’ conception of nature.

That itself should suffice: it is a creature that itself cannot be seen with the eye. Beyond that, I already explained that the eggs can in fact be seen with the eye, and we all know perfectly well about its reproduction, including even children. Okay, I think we’ve exhausted the topic.