- The source for עשה דוחה לא תעשה
The Gemara on the bottom of 3b asked how we know that an עשה is דוחה a לא תעשה, and answered that we see this rule from the passuk that discusses the איסור of kelayim. The passuk states : “"לא תלבש שעטנז – You may not wear shaatnez, and the next passuk states "גדילים תעשה לך" – You should wear tzitzis. These two mitzvos are סמוכים – adjacent, and therefore teach us that one may attach tzitizis of wool onto a בגד of linen, even though this would be kelayim. This shows us that an עשה (tzitzis) pushes off a לא תעשה (kelayim). Rabbi Elazar then brings the passuk which is the source for making a derashah with סמוכים.
- When we make a derashah from סמוכים
The Gemara says that even Rebbe Yehuda, who does not darshen semuchim in other parts of the Torah, agrees that in sefer Devarim we do darshen semuchim. This is because in sefer Devarim the pesukim are either מוכח - evident by placement of the passuk that it is meant for a derashah, or מופנה – superfluous. In the case of our derashah of kelayim and tzitzis, the Gemara shows how both reasons can be seen. It is indeed מוכח from the passuk that it was written for a derashah, because why were the words "גדילים תעשה לך" written here in Devarim 22; they should have been written together with the parsha of tzitzis in Bamidbar 15? Rather, we see that the Torah clearly wanted that a derashah be made with the passuk of tzitzis. So too, the derashah may be made being that there is something superfluous here. It says in Vayikra (19, 19) ובגד כלאים שעטנז לא יעלה עליך"”, - and a beged that is a mixture of shaatnez should not come upon you. Why does the Torah need to repeat the איסור of kelayim in Devarim 22? Rather, the passuk of לא תלבש שעטנז is extra, and therefore we know that it is meant to be used for a derashah.
- Why we need two passukim for kelayim
The Gemara stated that the passuk of “"לא תלבש שעטנז is superfluous, being that we already know that kelayim is forbidden from the passuk "ובגד כלאים שעטנז לא יעלה עליך" . The Gemara then asks on this, that the passuk of לא תלבש שעטנז is not extra, because if we only had the other passuk, we would assume that it is forbidden to wear kelayim even if one does not intend to receive any הנאה from it. This assumption would come from the wording of the passuk "לא יעלה עליך" – Shaatnez may not be upon you, which we would understand to mean that if the kelayim are upon a person, meaning, a case where one is selling kelayim and the clothes are lying on top of him, then he is being עובר on kelayim, even though he has no intention of benefitting from wearing the clothes. Therefore, the second passuk of "לא תלבש" teaches that wearing the kelayim must be similar to wearing clothes. In order to be עובר, the wearer has to intend to receive pleasure from it. The Gemara continues, that if we would only have the passuk of לא תלבש, we would understand that one is only עובר on shaatnez if he wears it, as this gives him a lot of pleasure, but merely sleeping with a blanket that has shaatnez, which does not give much pleasure, is permissible. Therefore, we need the other passuk of “"לא תעלה to teach us that this too is forbidden. The Gemara discusses if both pesukim are indeed needed.
