The heifer’s ashes, combined with a mixture of other substances, were sprinkled upon a person who had become ritually impure, but the procedure had the paradoxical effect of rendering impure the kohen who performed the ritual.
How can one and the same substance do two contradictory things?
The Midrash Tanchuma on Parashat B’shallach offers an analogy. Three things, it tells us, led the Israelites to complain.
The incense was part of the required ritual, yet it caused the deaths of Aaron’s sons Nadav and Avihu. The Ark was an essential feature of worship, yet when Uzziah touched it, he was struck down (II Sam. 6:7). The rod was a legitimate symbol of authority, yet it brought the plagues upon Egypt.
When Israel brought these three complaints to God, says the Tanchuma, the Almighty said in reply: “The incense brought death – but it also brought atonement on Yom Kippur. The Ark brought tragedy – but it also brought blessing to Oved-Edom the Gittite in his battles. The rod brought problems – but it also enabled Moses to perform miracles.”
The message is that nothing is necessarily sacred in itself. It depends on the purpose for which it is used.
The parah adumah brings purity when the ministering kohen sincerely desires to help a fellow human being, but if he worries too much about his own dignity it can bring him problems.
Taken more broadly, this concept represents any human achievement, especially ideas. One person can use a great idea to bring healing and light: another person can use the same idea to bring destruction and harm.