The 31st prohibition is that we are forbidden from practicing divination, i.e. to use any of the various ways of arousing one's faculty of making educated guesses1 [regarding the future]. All those who have the faculty of predicting the future before it occurs can do so only because they have a strong ability to make educated guesses that are accurate and come true in the majority of cases — they therefore estimate what will happen. Some people are superior to others in this, just as some are superior to others in other spiritual faculties.

Those who have this faculty of estimation must perform some action in order to arouse this faculty and strengthen its effect2. Some will continuously strike the ground with a stick, and cry out with strange shouts, and clear away their thoughts; after doing so for a long period of time, they go into a semi-conscious state and predict the future. I once witnessed this in the inner West.3

Some will throw small stones on a piece of leather, and then stare at them for a long period of time and then state their prediction. This is well known in every place I have passed through. Some throw a long leather belt onto the ground, and then stare at it and state their prediction.

The purpose of all these [actions] is to arouse the faculty within the person; not that the particular object affects anything or indicates anything. The common people are mistaken in this regard — when some of the predictions come true, they think that those actions showed the person what would be. This mistake reached the point4 where they thought that some of these actions actually caused what occurred. This is what those who study the paths of the stars5 believe, since this belongs to the same category, i.e., one of the ways of arousing this faculty. Therefore no two individuals will make the same prediction, even though they are equal in their knowledge of the rules [of astronomy].

One who performs any of these actions or any others similar to them is called a kosem [one who practices divination].

The source of this commandment is G‑d's statement (exalted be He),6 "Among you, there shall not be found anyone...who practices divination [kosem k'samim]."

In the words of the Sifri, "Who is considered a kosem? One who grasps a stick and says, 'Shall I go or not?'" It is regarding this method of divination which was popular at that time that the prophet said,7 "My people ask their stick, and their staff speaks to them."

One who transgresses this prohibition — i.e. who does the divination and makes his prediction through performing the particular action — is punished by lashes. One who asked the kosem the question, however, is [merely] very detestable.8

The details of this mitzvah have been explained in tractate Sanhedrin,9 and in Tosefta Shabbos,10 and in Sifri.