Commentary for Shabbat 135:9
אבל הכיר ולבסוף שכח מאי חייב על כל שבת ושבת אדתני היודע עיקר שבת ועשה מלאכות הרבה בשבתות הרבה חייב על כל שבת ושבת ליתני הכיר ולבסוף שכח וכ"ש הא מאי היודע עיקר שבת מי שהיה יודע עיקרה של שבת ושכחה
Rab and Samuel both maintain: Our Mishnah treats of a child who was taken captive among Gentiles, or a proselyte who became converted in the midst of Gentiles.<span class="x" onmousemove="('comment',' So that they never knew the laws of the Sabbath. ');"><sup>23</sup></span> But if one knew and subsequently forgot, he is liable [to a sin-offering] for every Sabbath.<span class="x" onmousemove="('comment',' He is regarded as knowing the sanctity of the Sabbath but forgetting on each occasion that it is the Sabbath. ');"><sup>24</sup></span> We learnt: HE WHO FORGETS THE ESSENTIAL LAW OF THE SABBATH: surely that implies that he knew [it] originally? — No: what is meant by HE WHO FORGETS THE ESSENTIAL LAW OF THE SABBATH? That the very existence of the Sabbath was unknown<span class="x" onmousemove="('comment',' Lit., 'forgotten'. ');"><sup>25</sup></span> to him. But what if he knew and subsequently forgot; he is liable for every Sabbath? Then instead of teaching, HE WHO KNOWS THE ESSENTIAL LAW OF THE SABBATH AND PERFORMS MANY LABOURS ON MANY SABBATHS, INCURS A SIN-OFFERING ON ACCOUNT OF EACH SABBATH: let him teach, He who knew and subsequently forgot, and how much more so this one? — What is meant by, HE WHO KNOWS THE ESSENTIAL LAW OF THE SABBATH? That he who knew the essential law of the Sabbath and forgot it.