NUM197 It became clear to me that an act which is performed for God’s sake is, in essence, made up of [two things]: purity of heart and of mind. Any corruption of the inner intent renders the [“good”] act unacceptable, even if performed many times and on a regular basis. This we see from Scripture: “Even if you offer many prayers, I will not listen; your hands are full of blood. Wash yourselves, make yourselves clean, remove your evil deeds from My sight; cease to do evil (Yeshayahu 1:15-16); Rather, it is a thing very close to you, in your mouth and in your heart, that you can do parentheses (Devarim 30:14); My son, give me your heart, and your eyes-- let them keep to my ways parentheses (Mishlei 23:26).” Our sages have expounded this [last verse] as follows: “If you give Me your heart and your eyes, [then] I will know that you are [truly] Mine” (Yerushalmi, Berachos 1:5). Scripture says further: Do not stray after your heart and eyes (Bemidbar 15:39). …”