If an extraneous thought suddenly enters your mind [during prayer], you should feel humiliated, for they are pushing you out of the King’s palace. You should return to the palace with great shame and submission. For the sin of having foreign thoughts is like that of giving birth to an illegitimate child, as the Gemara states: “one sheep follows the other”⁷ 63a. I.e. the acts of the children follow the acts of the parents. For thought is masculine and feminine, and voice and speech are masculine and feminine.⁸ If you have a foreign thought and speak words of holiness, those words are like a bastard, whose body is normal, but whose soul is bad.⁹ So too, the speech uses holy letters, but the thought is bad; for by thinking about something else, you give birth to a bastard with these holy words. You should also imagine that G‑d is asking you, “Why have you come into the word, while I am not in the word?”– for you thoughts are in other things. Tzava’as HaRivash, p. 10b¹⁰, and the end of the sefer Ohr Torah, and Likutim Amarim, p. 18b.

footnotes: ⁷ Kesuvos ⁸ Voice is non-differentiated sound, and is considered masculine. Speech gives it shape and meaning, and is considered feminine. This follows the principle that the vessel – the feminine aspect – gives form to the expansive masculine element. ⁹ The Sages consider the soul of an illegitimately conceived individual to be blemished by the sinful act of his parents. ¹⁰ See also Likutey Agados