WHEN MAY ONE LEARN OTHER CHACHMOS? [Chachmah: secular]
The gestation period of snakes is seven years. Nothing corresponds to this cursed animal;
(Rav Yehudah): We learn from "Arur Atah mi'Kol ha'Behemah umi'Kol Chayas ha'Sadeh."
Question: If the snake was cursed more than Behemos, all the more so it is cursed more than Chayos!
Answer: Rather, just like a Behemah is cursed seven times more than a Chayah (the gestation of a donkey is 12 months, and that of a cat is 52 days), the snake was cursed seven times more than a Behemah, i.e. seven years.
Kaisar: The elders of Athens mated snakes, and a snake was born after three years!
R. Yehoshua: The mother was already four years pregnant before they mated them.
Kaisar: The elders of Athens are very wise! (Surely, they did not err.)
R. Yehoshua: Chachmei ha'Torah are wiser.
Shabbos 116b (Mishnah): We may not read (Kisvei ha'Kodesh on Shabbos, for this draws people away from the lecture in the Beis Medrash).
(Rav): It is forbidden only at the time of (the lecture in) the Beis Medrash. At other times one may read them;
(Shmuel): It is forbidden even at other times.
(Rav Ashi): Shmuel holds like R. Nechemyah;
(Beraisa - R. Nechemyah): Chachamim forbade reading Kisvei ha'Kodesh in order that people will reason that if one may not read Kisvei ha'Kodesh, all the more so secular documents are forbidden!
Eruvin 41b (Mishnah): Once, Chachamim were on a boat. It entered the port on Shabbos. R. Gamliel ruled that they may get off, for he saw that they were within the Techum before dark.
43b (Beraisa): R. Gamliel had a hollow reed. With it he could see (exactly) 2000 Amos on land or at sea.
Menachos 99b - Question (Ben Dama, R. Yishmael's nephew): May someone like me, who learned the entire Torah, learn Chachmas Yevanis? (This is Greek wisdom, or a crafty way of speaking.)
Answer (R. Yishmael): "Lo Yamush Sefer ha'Torah ha'Zeh mi'Picha v'Hagisa Bo Yomam va'Laylah." If you find a time that is neither day nor night, at that time you may learn Chachmas Yevanis.
This is unlike R. Shmuel bar Nachmani;
(R. Shmuel bar Nachmani): "Lo Yamush..." is not an obligation or Mitzvah. It is merely a blessing.
Kidushin 82a (Mishnah - R. Nehurai): I will not teach my son any profession, only Torah, for one enjoys its reward in this world, and the principal remains intact for the next world.
Makos 8b: The Mishnah says that a father can get Galus for killing his son. The case is, he was teaching him carpentry when he killed him. (This is not a Mitzvah.)
Question: It is a Mitzvah to teach one's son a trade!
Answer: The case is, the son already knew a trade.
Perush ha'Mishnayos (Shabbos 23:2): On Shabbos and Yom Tov one may learn only prophecies and explanations of them.
Ba'al ha'Ma'or (Shabbos 43a DH v'Ha): All agree that one may not read documents of commoners on Shabbos. The custom spread to expound every Shabbos until the meal, therefore the entire day we do not refrain from anything with Kedushah.
Ran (Reish 43b): Shmuel holds like R. Nechemyah, that Chachamim forbade reading Kisvei ha'Kodesh so that people will reason that all the more so, secular documents are forbidden!
Rashba (1:772): On Shabbos, one may look in a telescope. It is a Kli. It is no worse than a Sefer of Chachmah. Reading what is engraved on copper plates is no worse than reading what is written in a Sefer.
The questioner (in Teshuvas ha Rashba 4:102): A proof for this is from R. Gamliel's reed.
Note: I do not understand his proof. R. Gamliel looked before Shabbos! Even if he looked Bein ha'Shemashos, perhaps R. Gamliel holds like Rebbi, who permits mid'Rabanan Isurim of Shabbos Bein ha'Shemashos.
Rashba (7:288): One may read medical books on Shabbos, for they are Chachmah. Also the Ramban says so. This is unlike the Isur to read commoners' documents. Those are records or debts.
R. Bechaye, introduction to the Torah (DH Derech ha'Shelishi): Our Torah includes all Chachmos. Other Chachmos are based on investigation. Our Torah is from Hash-m.
Maharsha (8b DH b'Agadas): Yisrael have a higher Chachmah than all the Nochrim. All their investigations are merely of nature. It is not Divine law. Chachmei Nochrim thought that the gestation of a snake is three years. Based on its nature it should have been three years, like other Chayos, but according to the Torah and Divine wisdom, it was cursed for its evil.
Poskim and Acharonim
Shulchan Aruch (OC 307:17): On Shabbos and Yom Tov one may learn only Divrei Torah. Other Chachmos are forbidden. Some permit. According to them, one may look in a telescope on Shabbos (and adjust it and move it).
Beis Yosef (DH Korin): The Ba'al ha'Ma'or and Ran connote that we read on Shabbos only matters with Kedushah.
Mishnah Berurah (65): The custom is to be lenient. Eliyahu Rabah says that one who Yir'as Shamayim should be stringent, because the Rambam and Ran forbid.
Mishnah Berurah (66): There is no difference between understanding Chachmah through a Sefer or through a Kli.
Kovetz Shi'urim (2): Even if one need not read books of Apikorsus (heresy), but he can learn the Chachmah only in a university among Nochrim and this will lead to ties with Nochrim, it is forbidden due to ""Hishamer Lecha Pen Tinakesh Achareihem." If neither of these apply, and he learns it for the sake of his profession, it is permitted. On Shabbos, one may discuss hiring a teacher to teach a trade to his son, for this is desires of Shamayim (OC 306:6). It is a Mitzvah (Mishnah Berurah 306:30). This is for most people. If one sees that his son desires Torah and has aptitude for greatness in Torah, he should teach him only Torah, like R. Nehurai taught. Surely R. Nehurai did not mean that everyone should learn only Torah. Until Moshi'ach comes, this cannot be! The Gemara asked that it is a Mitzvah to teach one's son a trade, and answered that the son already knew a trade. It did not say that the Mishnah is like R. Nehurai, for all agree that it is a Mitzvah.
Kovetz Shi'urim (3): The Yerushalmi inferred that R. Yehoshua, who forbids learning Greek due to "v'Hagisa...", forbids learning a trade. It answered that this was a special decree. This shows that the Isur to learn other things is not due to Bitul Torah. Why didn't the Gemara answer that the case is, he already knows a trade, so he may not learn another trade due to Bitul Torah? It gave an answer that applies to everyone. If one wants to learn other Chachmos for pleasure, not for a trade, it is proper to forbid due to Bitul Torah. The Rema (below) discusses this. One may learn them, but they must be secondary. If this is due to Bitul Torah, perhaps this applies only to men. However, also women must know Mitzvos that they are commanded about. Or, perhaps the Isur is making it seem that Chachmos have importance resembling Torah.
Rema (YD 246:4): One should not learn other Chachmos. One may learn other Chachmos on occasion, as long as it is not Sifrei Minim.
Gra (17): Menachos 99b forbids learning other Chachmos. The Sifri (va'Eschanan 34) expounds "v'Dibarta Bam" - make them (Divrei Torah) primary, and not secondary. Your engagement should be only in them.
Maharal (Nesivos Olam, Nesiv ha'Torah 14 DH v'Davar): We heed what Nochri Chachamim say about what is below (but not above) the sphere of the moon, for they are Chachamim of nature. One should learn such Chachmah, for Hash-m gave it. One who sees a Nochri Chacham blesses "Who gave from His Chachmah to flesh and blood (Berachos 58a). We do not rely on what they say about above the moon, for that is Chachmah of Elokus. Do not say that one should not learn it, due to "v'Hagisa". The Gemara forbids this regarding Chachmas Yevanis. This is because it has no use for Torah. Chachmah that helps understand the world is like a ladder to ascend to Chachmah of Torah. One may learn it.
Derech Chachmah of the Ramchal (DH Rebbi - Emnam): There is Chachmah of the Torah, and Chachmah of logical investigation. One must learn it in order that he can attain proper understanding of Kedushah, just like a craftsman cannot make anything without his tools. It is not intrinsically important, just it happens to be a means. It has two branches. One is what is needed in order to fulfill Mitzvos. The other branch is matters such as geometry, arithmetic and astronomy, which is needed for calculation of Ibur (leap years or months), Kil'ayim, Techumim, etc. It a practical need, and also a Mitzvah, to learn a trade. Chazal expounded "you will choose life" - this is a trade. One might have thought that he can sit idle - "b'Chol Mishlach Yadecha Asher Ta'aseh" teaches otherwise. One picks a trade, and learns everything needed for it. E.g. one who decides to be a doctor must learn nature and dissection and everything needed for this profession. If one needs to go among Chachmei Nochrim, it is proper to learn what will honor him in their eyes, and this will be a Kidush Hash-m. Chachmos such as geometry and arithmetic that are needed for the sake of Mitzvos or Dinim or a trade, one should learn only as much as is needed. If a Chachmah is not needed for any of these, but he would enjoy knowing it, this is like trips that people take for enjoyment. He should overcome his desire, just like one should overcome his desire for excess food and trips.