pidaya kriyate tapah
tat tamasam udahrtam
mudha—foolish; grahena—with endeavor; atmanah—of one's own self; yat—which; pidaya—by torture; kriyate—is performed; tapah—penance; parasya—to others; utsadanartham—causing annihilation; va—or; tat—that; tamasam—in the mode of darkness; udahrtam—is said to be.
And those penances and austerities which are performed foolishly by means of obstinant self-torture, or to destroy or injure others, are said to be in the mode of ignorance.
There are instances of foolish penance undertaken by demons like Hiranyakasipu, who performed austere penances to become immortal and kill the demigods. He prayed to Brahma for such things, but ultimately he was killed by the Supreme Personality of Godhead. To undergo penances for something which is impossible is certainly in the mode of ignorance.
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