Q&A: A Logical Fallacy?
A Logical Fallacy?
Question
As you know, the mistakes of second-order rather than first-order halakhic ruling are not only about the wedding ring, but also that according to the Talmud, parchment that is not treated with gallnuts is invalid; it is merely daphtera. And only among our brethren, who among other things did not have gallnuts available, did they switch to lime treatment, what is called in Arabic “fragment at rik” (which the Geonim explicitly say is invalid, and so does Maimonides in a responsum).
One of Rabbenu Tam’s “proofs” for permitting it is Tosafot on Sabbath 79b: “In the first chapter of Tractate Gittin (11a), regarding a document: ‘But don’t we require something that cannot be forged?’ And it answers: ‘Because it was treated with gallnuts.’ That implies that if it was not treated with gallnuts, it can be forged and is invalid. And we are witnesses that ours cannot be forged; therefore it is considered like gall-treated parchment.”
Is that a logical argument? If gall-treated hide cannot be forged, does that mean that everything that cannot be forged is equivalent to gall-treated hide? If all cucumbers are green, does that mean that everything green is a cucumber?
Answer
Absolutely not a logical mistake. A completely valid argument. In logic, if one assumes an implication of the form A -> B, then one may derive from it the negation of the consequent:
~B -> ~A
That is equivalent to it. And that is exactly what Tosafot did here.
And in your example: if all cucumbers are green, then whatever is not green is not a cucumber.
Discussion on Answer
But there is no typo in Tosafot, and Tosafot says what I wrote. Completely valid.
No, Rabbi Michi (what happened—why did you show up as Rabbi David-Michael Abraham? I hope this lengthening signifies the trait of being slow to anger).
What does Gittin have to do with validating a Torah scroll? That is what I meant.
You said that if all cucumbers are green, then everything that is not green is not a cucumber.
But Tosafot says
that if all cucumbers are green, then the green thing I have is a cucumber?
Meaning, if gall-treated hide cannot be forged, does that necessarily mean that everything that cannot be forged is considered gall-treated? No. Only for the purposes of Gittin, where it is enough to meet the condition that it cannot be forged, then anything that cannot be forged is sufficient.
I did not get into the topic at all. The quote from Tosafot that you brought is this:
“If, were it not treated with gallnuts, it could be forged and would be invalid; and we are witnesses that ours cannot be forged; therefore it is considered like gall-treated parchment.” That is completely valid.
I understand. Right.
It is considered like gall-treated parchment for the purposes of Gittin, where that is the determining parameter. But from here to validate it for Torah scrolls, tefillin, and mezuzot?
That is the logical fallacy.
There was a typo in what I wrote, and the correction is this: the fact that gall-treated hide cannot be forged does not mean that everything that cannot be forged has the same status as gall-treated hide, regarding validity for Torah scrolls, tefillin, and mezuzot. It only means that everything that can be forged is not gall-treated hide. In other words, it does not mean that Rabbenu Tam’s parchment, which cannot be forged, also has the status of gall-treated hide for the laws of Torah scrolls, tefillin, and mezuzot.
That is, if all cucumbers are green (all gall-treated hides cannot be forged), then certainly whatever is not green (that is, whatever can be forged) is not a cucumber (is not like gall-treated hide for Torah scrolls, tefillin, and mezuzot). But that does not mean that everything green (any parchment that cannot be forged) is a cucumber (has the status of gall-treated hide for Torah scrolls, tefillin, and mezuzot).