"And its owner did not guard it." If the owner tied it with a rope and shut it in properly, yet it got out and caused damage whether harmless or forewarned he is liable; these are the words of Rabbi Meir. Rabbi Yehuda says: for a harmless ox he is liable, but for a forewarned ox he is exempt, as it is said, "and its owner did not guard it" and this was indeed a guarding. Rabbi Elazar says: it has no proper guarding except the knife [slaughter]. What is Rabbi Meir's reason? He holds that oxen in general are not presumed to be guarded, and the Merciful One said the harmless ox's owner is liable so that he should provide it lesser guarding; then the Merciful One repeated "and its owner did not guard it" regarding the forewarned ox, so that he should provide it superior guarding and he learns goring for the harmless from goring for the forewarned. Rabbi Yehuda holds that oxen in general are presumed to be guarded, and the Merciful One said the harmless ox pays so that he should provide superior guarding; then the Merciful One repeated, and so on this is one amplification after another, and an amplification after an amplification serves only to limit, so the Merciful One limited the requirement of superior guarding. And if you say that goring for the harmless and goring for the forewarned should be compared, the Merciful One limited it with "and its owner did not guard it" for this one and not for another. But is this phrase not needed for the prohibition itself? If so, let the Merciful One have written "and he did not guard"; what does "and its owner did not guard it" teach? For this one and not for another. It was taught, Rabbi says: whether harmless or forewarned, if they guarded it with lesser guarding, the owner is exempt. What is his reason? He holds like Rabbi Yehuda, who said that lesser guarding suffices for the forewarned ox, learning goring for the harmless from goring for the forewarned.
What Counts as Failing to Guard the Ox for Tam and Muad
Yalkut Shimoni on Torah 340:2
וְלֹא יִשְׁמְרֶנּוּ בְּעָלָיו, קְשָׁרוֹ בְּעָלָיו בְּמוֹסֵרָה וְנָעַל בְּפָנָיו (שֶׁלֹּא) כָּרָאוּי וְיָצָא וְהִזִּיק, אֶחָד תָּם וְאֶחָד מוּעָד, חַיָּב, דִּבְרֵי רַבִּי מֵאִיר. רַבִּי יְהוּדָה אוֹמֵר, תָּם חַיָּב וּמוּעָד פָּטוּר שֶׁנֶּאֱמַר וְלֹא יִשְׁמְרֶנּוּ בְּעָלָיו וְשִׁמּוּר הוּא זֶה. רַבִּי אֶלְעָזָר אוֹמֵר, אֵין לוֹ שְׁמִירָה אֶלָּא סַכִּין. מַאי טַעְמָא דְּרַבִּי מֵאִיר, קָסְבַר סְתָם שְׁוָרִים לָאו בְּחֶזְקַת שִׁמּוּר קָיְימֵי, וְאָמַר רַחֲמָנָא תָּם חַיָּב, דְּלֵיעֲבִיד לֵיהּ שְׁמִירָה פְּחוּתָה, הֲדַר אֲמַר רַחֲמָנָא וְלֹא יִשְׁמְרֶנּוּ גַּבֵּי מוּעָד, דְּנֵעֲבִיד לֵיהּ שְׁמִירָה מְעֻלָּה, וְיָלִיף נְגִיחָה לְתָם נְגִיחָה לְמוּעָד. וְרַבִּי יְהוּדָה סְבַר, סְתָם שְׁוָרִים בְּחֶזְקַת שִׁמּוּר קָיְימֵי, אֲמַר רַחֲמָנָא תָּם מְשַׁלֵּם, דְּנֵעֲבִיד לֵיהּ שְׁמִירָה מְעֻלָּה, [הֲדַר אָמַר רַחֲמָנָא וְכוּ'], הֲוֵי רִבּוּי אַחַר רִבּוּי וְאֵין רִבּוּי אַחַר רִבּוּי אֶלָּא לְמָעֵט, מִיעֲטֵיהּ רַחֲמָנָא לִשְׁמִירָה מְעֻלָּה. וְכִי תֵימָא, נְגִיחָה לְתָם נְגִיחָה לְמוּעָד, הָא מִעֵט רַחֲמָנָא וְלֹא יִשְׁמְרֶנּוּ לְזֶה וְלֹא לְאַחֵר. וְהָא מִבַּעְיָא לֵיהּ לְלָאו, אִם כֵּן נִכְתּוֹב רַחֲמָנָא וְלֹא יִשְׁמֹר, מַאי וְלֹא יִשְׁמְרֶנּוּ, לְזֶה וְלֹא לְאַחֵר. תַּנְיָא, רַבִּי אוֹמֵר, אֶחָד תָּם וְאֶחָד מוּעָד שֶׁשָּׁמְרוּ שְׁמִירָה פְּחוּתָה, פָּטוּר. מַאי טַעְמָא, סְבַר לָהּ כְּרַבִּי יְהוּדָה, דַּאֲמַר מוּעָד בִּשְׁמִירָה פְּחוּתָה סַגִּי לֵיהּ, יָלִיף נְגִיחָה לְתַם וּנְגִיחָה לְמוּעָד.