Why are a minor (Sabiyy) and an insane person (Majnun) exempt from the obligation of Hajj?

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A minor and an insane person are not accountable (Mukallaf). This is established by the Hadith narrated by Ali ibn Abi Talib, that the pen (of accountability) is lifted from three: the sleeper until he wakes, the minor until he reaches maturity, and the mentally afflicted until he regains his senses. This Hadith is graded as Hasan.