The Me’iri points out that during Talmudic times there were popular beliefs in destructive forces, amulets, etc. – ideas that today would be considered superstition. As long as these beliefs did not involve Avoda Zara or actual witchcraft, the Sages made no attempt to convince the people that they were untrue. This was certainly true in cases where these beliefs were so strong that the psychological belief would cause a physiological reaction to a given circumstance. The Gemara’s formulation of this appears at the end of the discussion here, which recognizes that those who are concerned about such things should be concerned, but those who are not particular about them do not need to worry.
One of these beliefs was the danger of zugot – that is to say, that doing things in pairs was hazardous. This concern leads to a question being raised about the Seder night. How can the Sages obligate participants to drink four cups of wine, when doing so would be involving oneself in zugot?
Rav Nahman said that the verse said: “It was a night of watching to the Lord” (Shmot 12:42), which indicates that Passover night is a night that remains guarded from demons and harmful spirits of all kinds. Therefore, there is no cause for concern about this form of danger on this particular night.
Rava said a different answer: The cup of blessing for Grace after Meals on Passover night is used in the performance of an additional mitzva and is not simply an expression of freedom. Therefore, it combines with the other cups for the good, i.e., to fulfill the mitzva to drink four cups, and it does not combine for the bad.
The lengthy discussion of zugot in our Gemara includes a conversation between Rav Pappa and Yosef the Demon [Shida] about the respective dangers of one set of zugot (two) and two sets of zugot (four).
The identity of Yosef Shida, who appears in a number of stories throughout the Gemara, is not clear. Rashi brings two possible explanations, one which sees him as a person who was an expert in shedim (demons) and the occult, while the second suggests that he was, himself, a demon with whom the Sages developed a relationship to the extent that they discussed issues of shedim with him. Either one of these explanations can be supported by the various stories about shedim that appear in the Gemara.