The Rambam: It is a clear and apparent matter that the concepts of purity and impurity are Scriptural decrees and they are not matters determined by a person’s understanding and they are included in the category of chukim. Similarly, immersion in a mikveh to ascend from impurity is included in the category of chukim, because impurity is not mud or filth that can be washed away with water. Instead, the immersion is a Scriptural decree and requires the focusing the intent of one’s heart. Therefore our Sages said: "When one immersed, but did not intend to purify himself," it is as if he did not immerse. Although it is a Scriptural decree, there is an allusion involved: One who focuses his heart on purifying himself becomes purified once he immerses, even though there was no change in his body. Similarly, one who focuses his heart on purifying his soul from the impurities of the soul, which are wicked thoughts and bad character traits, becomes purified when he resolves within his heart to distance himself from such counsel and immerse his soul in the waters of knowledge. And Ezekiel 36:25 states: "I will pour over you pure water and you will be purified from all your impurities and from all your false deities, I will purify you."