However, there were later customs that had clear reasons associated and those reasons no longer apply. Are they the only reasons for these customs? I do not know. They are the only stated reasons, and they were not forbidden in the Torah before the rabbis forbid them, so I have no information to say otherwise. I like to go by transparent sources, meaning those which were available to every community, rather than some source from a particular rabbi in some corner of the world. There was a school that began kabalah in Tzfat, which was responsible for the spiritualization of some very practical measures as well as plain old syncratism. Many of these customs irk me, and modern rabbis, “poskim” and even “gedolim” are too weak and pusillanimous to examine them, by their own admission.
1) Kaparos using live animals – All evidence shows that this primitive custom began in the Ashkenazic world as a non-Jewish practice of magic. It was forcefully rejected as such in the rational Sephardic world of rishonim in strong terms, and revived without transparent source by endorsement of the kabbalist Sephardic group in the Middle Ages, so it has spiritual street cred, even though most lay scholars are familiar with its sordid history. The strictest practitioners today use a separate chicken for each family member, hens for girls and roosters for boys. In the ultimate fence around the magic, pregnant woman use a hen plus another of each sex, for the baby, even if the baby’s sex is known by modern means. Sympathetic magic meets rabbinic OCD, anyone?
2) Avoiding Kiddush between 6 and 7 – This is not even internally consistent, nor consistent with the sources, as I discussed before. Its ultimate source is non-Jewish astrological religious superstition, and even quoted as such. It is so nonsensical, in fact, that there are no rational explanations for the custom or even how it is supposed to be practiced. In Chabad today, the custom is practiced prolifically, based on each individuals understanding of the strange internal inconsistencies, even though it has no clear basis in Chabad custom. It is only mentioned in the SA-SZL as a custom that some people had.
3) Chalav Yisroel – In more agricultural times, it was profitable to mix unsalable milk from a non-kosher animals with milk from kosher animals to increase the volume without increasing expense. These products had to be made quickly into cheese or butter, as there was no preservative methods that would maintain liquid milk to market. According to the sages, the non-kosher milk would not congeal and easily separated from the cheese at that point. Therefore, they forbid Jews to drink any milk they might be able to obtain, unless a trusted Jew had supervised the process to make sure it wasn’t bulked with non-kosher milk. Non-Jewish cheese or butter was OK, because any rennet used was nullified in amount and taste, and non-kosher milk would not congeal. In the time that this custom was invented, there was no great inconvenience, because people did not buy and sell milk at market, and only traded it in small private transactions, if at all. Milk became a staple in the US with prevailing methods to make milk safe and preservable. Thus, in the last 100 years or so, it became an issue. One of the primary rabbis of the 20th century, Rabbi Moshe Feinshtein, allowed people to use milk for their children as it became a widespread complaint, based on the fact that it is no longer profitable to bulk milk with non-kosher milk, given other better methods of bulking and the fact that such practice was regulated by the government and a violator would risk fines, making it further unprofitable for a commercial dairy to do.
Today, people keep chalav yisroel out of adherence to these old taboos of a yesteryear economy, even cheese and butter, which the sages permitted. (The Sifsei Kohen also brings an objection that milk from treif cows might be mixed in and that might forbid all cheese and butter as well. However, AFAIK, mashgichim are not examining each cow today.) In communities outside of big cities, milk products are difficult to obtain and far more expensive than their counterparts. There are many more products that use dairy today, and thus it is far more inconvenient than the rabbis ever dreamed of when they instituted this rule. Today, all candy and baked goods, all products with cheese or cheese flavoring, and many other products that people would use daily are disqualified. Worse yet, kashrus issues stemming from exposure to non-chalav yisroel, like using dairy equipment, even if the food itself is not dairy, is also disqualified. Defenses for the custom range from defenses that it is still applicable somehow, and that RMF only allowed it narrowly etc. to full spiritualization. It needs to be revised for the modern world, if not dropped altogether, especially in the US.
4) Pas yisroel and bishul yisroel – The laws that bread must be baked by a Jew or that staple foods must be cooked by a Jew were instituted to prevent fraternization with local non-Jews, and possible intermarriage. People made baked goods from their homes, as they did not preserve well to market, and the law was aimed at sharing with the neighbor. By the times and places where there were commercial bakeries in city centers, places that had them allowed people to patronize them, with stipulations, since a professional baker was selling a product, rather than fraternizing. IOW, they recognized the increased inconvenience of the custom and the fact that its reasoning did not apply to them and they allowed it. (This would counter the spiritualization and “other reasons” claims.) Bread is always considered a staple. Cooked items, though, can be differentiated based on their importance. Obviously, if the goal was preventing intermarriage, the foods affected would be those that were impressive to a guest.
Today, almost all baked goods trade is done from commercial bakeries. However, all manner of products are available for sale, as well. What was viewed as a mechanism to keep separation in a shtetl now forbids people from using any local bakery or buying staple products off the shelf. Pas Yisroel, when available, is far higher cost for the same products. Many processed goods are cooked and marketed now, something that did not happen with great regularity on the old days. In response, in the Ashkenazic world many centuries ago, some modifications were made, such that even a small addition, the lighting of the fire by a Jew or the stirring of the pot, was enough to permit the food, since it thwarted the reason for the prohibition (an unsupervised kitchen), with minimal inconvenience for commercial enterprises. In the realm of cooked foods, there are those who forbid potato chips and Cheerios without the bishul hechsher. These are clearly out of the original intention of the law, as it uses the term “presented on the table of the king”, to describe how important it needs to be to fall under restriction.
5) Drawing water for Mayim Shlanu at shkiah – The water that is used for shmurah matzah must be as cold as possible to allow the greatest protection from becoming chametz. According to the Gemorrah and later halacha sources, this time is sunset. The reason is that all day the sun is above us and warming the earth and then at night it goes under the earth and warms the water from beneath the earth. The proof for this is that one can see steam rising from the water in the morning, showing that the sun made the water very hot all night from beneath. Therefore, the best time to draw water when it is perfectly cold is sunset. Nuf said.
6) Black hats and coats – Chabadniks and others today require a black fedora and a suit jacket with a special ceremonial belt to pray. This is because one should dress for prayer as if you were going before the king. This is, in fact, formal dress … if you are a European living in 1930 or one of the Blues Brothers.
7) Rabainu Tam tefilin - Rashi, Rabainu Tam, Ra'avad and other rishonim had different ideas about the arrangement of the parhios in the hand and head tefilin. The SA specifies the way of rashi, but there were various spiritualist rabanim in later times who wore several pairs, some at the same time. In Chabad, this has evolved into everyone wearing Rashi with a blessing and Rabainu Tam without.
Pesach requires its own post.