Qitzur Shulchan Arukh – 67:6
ו: אֵין הַנֵּדֶר חָל אֶלָּא אִם הָיָה פִּיו וְלִבּוֹ שָׁוִים . אֲבָל אִם נָדַר בְּטָעוּת, שֶׁלֹּא הָיָה דַּעְתּוֹ כְּמוֹ שֶׁהוֹצִיא בִּשְׂפָתָיו, אוֹ שֶׁהִרְהֵר בְּלִבּוֹ נֶדֶר וְלֹא הוֹצִיאוֹ בִּשְׂפָתָיו, אֵין זֶה נֵדֶר
The neder doesn’t come into existence unless his mouth and heart are the same. However, if he made a neder by accident, that his thought wasn’t like what came out of his mouth, or if he contemplated a neder in his heart but it didn’t come out of his mouth, it is not a neder.
That an unintentional oath, a misspeech, isn’t binding, shouldn’t surprise.
However, it is very Jewish that the reverse — an unexpressed thought — lack the same reality as an expressed one. In general, Judaism focuses on how we impact ourselves. The thought becomes more real and more ingrained once it’s acted upon.
