Red Heifer

 Tanchuma (Chukas 08) A certain convert questioned Rabban Johanan ben Zakkay, “These things which you do seem like a kind of sorcery. You bring a heifer, burn it, pound it, and take its ashes. Then when one of you is defiled by a corpse, they sprinkle two or three drops on him, and you say to him, ‘You are clean.’” He said to him, “Have you ever had a bad spirit of madness enter you?” He told him, “No.” He said to him, “Perhaps you have seen someone into whom a bad spirit has entered?” He told him, “Yes.” He said to him, “So what did you do for him?” He said to him, “They bring roots and burn them beneath him. Then they sprinkle water on the spirit, and it flees.” He said to him, “Let your ears hear what you are uttering with your mouth. Similarly is this spirit an unclean spirit. Thus it is stated  ‘and I will also remove the prophets and the unclean spirit from the land.’ They sprinkle the purifying water upon him, and he flees.” After the gentile had left, R. Johanan's disciples said to him, “Our master, you repelled this one with a mere reed of an answer. What have you to say to us?” He said to them, “By your lives, a corpse does not defile, nor does a heifer purify, nor does water purify. Rather, G-d has said, ‘I have enacted a statute for you. I have issued a decree, and you are not allowed to transgress against my decree.’” Thus it is written  “This is the statute of the Torah.” And for what reason are all the sacrifices male and female, while that one is only female? R. Ayyevu said, “It is comparable to the son of a female slave who defiled a king's palace. The king said, ‘Let his mother come and clean up the excrement.’ Similarly has the Holy One, blessed be He, said, ‘Let a heifer come and atone for the incident of the golden calf.’”

Bamidbar Rabbah (19:08) A certain idolater asked Rabban Yoḥanan ben Zakai: ‘These actions that you perform seem to be a type of sorcery. You bring a heifer, burn it, crush it, and take its ashes. One of you becomes impure from a corpse, one sprinkles upon him two or three drops, and you say to him: You are purified.’ He said to him: ‘Has a spirit of insanity never entered you?’ He said to him: ‘No.’ ‘Have you seen a person into whom a spirit of insanity has entered?’ He said to him: ‘Yes.’ He said to him: ‘And what do you do to him?’ He said to him: ‘We bring roots, smoke them beneath him, and sprinkle water on it, and it flees.’ He said to him: ‘Let your ears hear what you express from your mouth. The same is true of this spirit, this spirit of impurity, as it is written: “I will remove the prophets and the spirit of impurity from the land”. We sprinkle upon it the water of sprinkling, and it flees.’After he left, his students said to him: ‘You rebuffed this one with a reed. What do you say to us?’ He said to them: ‘As you live, it is not the corpse that impurifies, and it is not the water that purifies. Rather, G-d said: I instituted a statute, issued a decree; you are not permitted to violate My decree, as it is written: “This is the statute of the Torah.”’

Rambam (Mikvaos (11:12) 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.