Pesachim 110 - Bad even numbers
Consuming foods portions in even numbers opens one to the pranks of the demons and to the witchcraft of the humans. Some say that this is patently untrue, some - that duality negates the oneness of God, and thus by avoiding it, we can quietly affirm our beliefs, and yet others - that good and evil forces do exist in the world.
The Talmud asks, how could the Sages establish a Seder with four obligatory cups of wine to be drunk? Two of the answers are that the night of the Seder is guarded from all evil, or that each cup, being a separate mitzvah, does not combine with the others.
Rava was so careful with even numbers that he mentally counted the cups he drank by marking the beams of the roof in this house. Abaye, after he would drink the first cup, his mother would right away give him two more, one in each hand. One man divorced his wife, and she then married a shop owner. Every day he would come to this shop to drink wine, but was very careful to count his cups and never drank an even number. Once, however, he lost count after sixteen and left, having drunk an even number; she performed witchcraft, and he burst.
The rule though is that only one who is mindful about even numbers and demons can fall under their influence. Another rule is that witchcraft and demons lost their power. Finally, a third rule is that one still must be mindful of it. These three rules seem contradictory, but there may still be a resolution.
Art: Demon sitting by Mikhail Aleksandrovich Vrubel