The ultimate refuge for all sentient beings is the One True Dharma Realm; there is nothing beyond this. Taking the Three Refuges means taking refuge in the One True Dharma Realm—there is nothing else to take refuge in!
All sentient beings possess the Buddha-nature and thus can all attain Buddhahood. However, without encountering the sacred Dharma, without meeting the Three Jewels, and without practicing within this supreme Dharma, one will ultimately fail to attain Buddhahood. It is like having a treasure buried deep within the ground at home: without guidance from a noble person, one will never discover the treasure and enjoy the joy of wealth.
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