And it will be, that the maiden to whom I will say, ‘Lower your pitcher and I will drink,’ and she will say, ‘Drink, and I will also water your camels,’ her have You designated for Your servant, for Isaac, and through her may I know that You have performed loving kindness with my master.”. (Genesis 24:14)
Superstitious beliefs that events can be interpreted as omens or portents for the future was always anathema to Judaism. The Torah explicitly prohibits nichush the act of determining future actions or fortune based on omen. (Leviticus 19:26)
The Gemara (Sanhedrin 65b) gives examples of lo tenachashu – not to be superstitious, including someone who says that he will have a bad day if bread falls from his mouth while he is eating or if his walking stick falls from his hand. Saluting a lone magpie or throwing salt over one’s shoulder are good modern day examples and should certainly be avoided (especially if you don’t know who’s behind you).
Eliezer’s scheme seems to therefore transgress this prohibition and the Gemara (Chulin 95b) apparently supports this. However, Maimonides (d. 1204) and Rabbeinu Nissim (d. 1376) argue that Eliezer’s actions would have been prohibited had they been illogical with no connection between the action and the sign. Since the condition he set related directly to finding a kind, good natured girl fitting for Isaac, it was not considered superstitious but common sense and rational.
This would imply that refraining from walking under a ladder for fear of something falling down, or not opening an umbrella indoors for fear of hurting someone are not only permitted but advisable practices, whereas acting in the same way out of superstition would violate this commandment.
While Rabbi Moshe Isserlis (d. 1572) rules stringently like Maimonides and Rabbeinu Nissim, Rabbi Avraham ben David (d. 1198) writes that we are allowed to make our own signs provided they are not typical superstitions (Laws of Non-Jews 11:4).
In the words of the 20th Century American politician R.E. Shay, “Depend on the rabbit’s foot if you will, but remember it didn’t work for the rabbit.”