Makkos, Chapter 3, Mishnah 4. In the previous Mishnah we introduced the concept of a lav hanitak la'aseh: a prohibition for which the Torah provided a remedy through the fulfillment of a positive commandment - and one is not lashed for it.
Our Mishnah takes up this question with regard to the mitzvah of shiluach haken, sending away the mother bird. The Torah commands us to send away the mother and take the young, and it forbade taking the mother while the young are still in the nest. The question is: if a person took the mother and afterward sent her away - does that sending away sever the prohibition? Does the positive commandment of sending her away come to remedy the transgression, or is it simply an independent command? This is the subject of the dispute in the Mishnah.
The text of the Mishnah:
"HaNotel eim al habanim" - one who takes the mother bird while the chicks or the eggs are still in the nest. The Tanna'im disagree about his law:
"Rabbi Yehudah omer: lokeh v'eino m'shale'ach" - in his view, the command "shale'ach teshalach es ha'eim" - "you shall surely send away the mother" - is an obligation from the outset: to send away the mother and only then take the young. It is not a remedy for having taken her, and therefore one who took the mother has violated the prohibition, receives lashes for it, and does not send her away.
"VaChachamim omrim: m'shale'ach v'eino lokeh" - in their view, sending her away after having taken her repairs the negative commandment, and this is therefore a lav hanitak la'aseh. Accordingly, he sends away the mother and does not receive lashes.
The rule laid down by the Sages:
"Zeh haklal: kol mitzvas lo sa'aseh sheyeish bah kum aseh, ein chayav aleha" - any prohibition that is accompanied by a positive commandment that comes to remedy it does not carry lashes.
However, if a person created a situation in which he can no longer fulfill the positive commandment - for example, he took the mother and slaughtered her, so that there is no longer any way to send her away, or she died in his hands before he managed to send her away - he receives lashes even according to the Sages, since the prohibition was not in fact severed.
In summary: in this Mishnah, Rabbi Yehudah and the Sages disagree about the law of one who takes the mother bird together with her young. According to Rabbi Yehudah, the mitzvah of sending her away is a command from the outset and does not remedy the taking, and therefore he is lashed and does not send her away. According to the Sages, sending her away severs the prohibition, and therefore he sends her away and is not lashed. From here comes the rule that any prohibition which has a positive commandment attached to it does not carry lashes - provided that the person has not himself destroyed the possibility of fulfilling that positive commandment.