The Mishna and Gemara in Makkos [21a and b] says that one receives a separate set of malkos for wearing shaatnez for every warning that he receives even if he doesn't take off and put on the garment again. It is enough that the TIME for taking off and putting on the garment elapses [אפילו לא שהה אלא כדי לפשוט וללבוש חייב].
The Rishonim DON'T GET IT!
Where is the מעשה here?? One does not receive malkos unless there is a מעשה??
The Ritva further perplexes us by saying that כיון שהיה יכול לפשוט ואינו פושט ועומד במלבושו חשבינן ליה כאלו הוא לובשו עכשיו - Since he could have taken it off and remained enclothed, it is as if he is doing a מעשה now.
Whaaaaaaat??
Where is the מעשה?? He is perfectly passive???
[There is a more famous answer that the original מעשה of putting on the garment counts for now. But that is not what the Ritva is saying.]
