Q. In Pirkei Avot I have found a statement, “Every day a bat kol goes forth from Mount Horeb and says, ‘Woe to mankind for insulting the Torah'” (Avot 6:2). What is a bat kol?
A. It is a heavenly message, literally “the daughter of a voice”. Mentioned quite frequently in rabbinic literature, sometimes it is heard by an individual and sometimes by the community. Its status is lower than the ru’ach hakodesh, the Holy Spirit, and from the time the prophetic spirit ceased in Israel the bat kol was God’s means of communicating with mankind (Gemara Yoma 9a). It is called “daughter of a voice” because it is not the voice itself that is heard, but an echo.
A bat kol is said to have announced that Tamar was innocent, that Samuel had not derived personal advantage from his office, and that Solomon had judged correctly between the two women who each claimed the child (Makkot 23b).
There is much discussion as to whether a bat kol has the authority to decide halachah. After three years of conflict between Bet Shammai and Bet Hillel a bat kol declared, “Both these (views) and these are the words of the living God, but the halachah is according to Bet Hillel”, and the sages accepted this ruling (Eruvin 13b).
However as a general principle Rabbi Yehoshua argued that “the Torah is not in heaven” (Deut. 30:12) and therefore halachah depends on the voices of the sages and not a voice from heaven.
As a guiding principle when a person is uncertain about how to proceed in life, Rabbi Shefatiah says in the name of Rabbi Yochanan, “Whence do we know that we may avail ourselves of a heavenly voice? Because it says, ‘And your eyes shall hear a voice behind you, saying…. (Isaiah 30:21)” (Megillah 32a). This applies, however, only if one hears the voice in an unusual place and if it says, “yes, yes” or “no, no”, i.e. it pronounces the word twice.