Eruvin 39A

לא יטייל אדם על פתח מדינה כדי שיכנס למרחץ מיד, הדר ביה!

a person may not stroll at the entrance to the city toward the end of Shabbat or a Festival in order to enter a bathhouse immediately upon the conclusion of Shabbat, then Rabba would have retracted his statement with regard to an eiruv. This baraita indicates that even walking on Shabbat for something one needs after Shabbat falls into the category of prohibited preparation.

ולא היא, שמע ליה ולא הדר ביה. התם ⁠— מוכחא מילתא, הכא ⁠— לאו מוכחא מילתא היא.

The Talmud rejects this argument: And this is not correct. Rabba heard this baraita but did not retract his ruling, as a distinction can be drawn between the cases. There, in the baraita pertaining to one who walks to the end of his field, or one who strolls at the entrance to the city, it is clear to all observers that he is doing so in order to determine what work the field needs after Shabbat, or to enter the bathhouse immediately after Shabbat, respectively. Whereas here, with regard to an eiruv, it is not clear to others that one’s actions are for the purpose of establishing an eiruv.

אי צורבא מרבנן הוא, אמרינן: שמעתא משכתיה. ואי עם הארץ הוא, אמרינן: חמרא אירכס ליה.

The Talmud explains: If he is a Torah scholar [tzurva merabbanan], we, the observers, would say: Perhaps his study pulled him, i.e., he was engrossed in his study and was not paying attention to where he was going. And if he is an am ha'aretz, we would say: Perhaps he lost his donkey and went to look for it. His actions give no indication that he is going to establish an eiruv for the following day, as establishing an eiruv does not require any recognizable action.