Rabbi Yitzchak and the school of Rabbi Yishmael teach that blessing is found only in matters concealed from the eye, and the Sages rule that one measuring grain should bless before measuring begins; if the blessing is recited after measurement is complete, it is a prayer made in vain, since blessing cannot rest upon what has been weighed, measured, or counted.
וְאָמַר רַבִּי יִצְחָק: אֵין הַבְּרָכָה מְצוּיָה אֶלָּא בְּדָבָר הַסָּמוּי מִן הָעַיִן, שֶׁנֶּאֱמַר: ״יְצַו ה׳ אִתְּךָ אֶת הַבְּרָכָה בַּאֲסָמֶיךָ״. תָּנָא דְּבֵי רַבִּי יִשְׁמָעֵאל: אֵין הַבְּרָכָה מְצוּיָה אֶלָּא בְּדָבָר שֶׁאֵין הָעַיִן שׁוֹלֶטֶת בּוֹ, שֶׁנֶּאֱמַר: ״יְצַו ה׳ אִתְּךָ אֶת הַבְּרָכָה בַּאֲסָמֶיךָ״. תָּנוּ רַבָּנַן: הַהוֹלֵךְ לָמוֹד אֶת גּוֹרְנוֹ, אוֹמֵר: ״יְהִי רָצוֹן מִלְּפָנֶיךָ ה׳ אֱלֹהֵינוּ שֶׁתִּשְׁלַח בְּרָכָה בְּמַעֲשֵׂה יָדֵינוּ״. הִתְחִיל לָמוֹד, אוֹמֵר: ״בָּרוּךְ הַשּׁוֹלֵחַ בְּרָכָה בַּכְּרִי הַזֶּה״. מָדַד וְאַחַר כָּךְ בֵּירַךְ – הֲרֵי זֶה תְּפִילַּת שָׁוְא, לְפִי שֶׁאֵין הַבְּרָכָה מְצוּיָה לֹא בְּדָבָר הַשָּׁקוּל וְלֹא בְּדָבָר הַמָּדוּד וְלֹא בְּדָבָר הַמָּנוּי, אֶלָּא בְּדָבָר הַסָּמוּי מִן הָעַיִן, שֶׁנֶּאֱמַר: ״יְצַו ה׳ אִתְּךָ אֶת הַבְּרָכָה בַּאֲסָמֶיךָ״.
And Rabbi Yitzḥak says: Blessing is found only in a matter concealed from the eye, as it is stated: “The Lord will command blessing with you in your storehouses” (Deuteronomy 28:8), where the grain is concealed. The school of Rabbi Yishmael taught: Blessing is found only in a matter over which the eye has no dominion, as it is stated: “The Lord will command blessing with you in your storehouses.” The Sages taught: One who goes to measure the grain on his threshing floor recites: May it be Your will, O Lord, our God, that You send blessing upon the product of our hands. If one began to measure the grain he says: Blessed is He Who sends blessing upon this pile of grain. If one measured and afterward recited this blessing, this is a prayer made in vain, because blessing is found neither in a matter that is weighed, nor in a matter that is measured, nor in a matter that is counted. Rather, it is found in a matter concealed from the eye, as it is stated: “The Lord will command blessing with you in your storehouses.”