“What is the great ocean
most difficult to leave forever?
The three realms of cyclic existence,
which toss in waves of pain.”
– Kelsang Gyatso, the seventh Dalai Lama
The “three realms of cyclic existence” refers to the three dimensions of unenlightened life, known as the realm of the senses(desire), the realm of form, and realm of formlessness. These realms are only unenlightened, of course, when experienced by an unenlightened being. They are often referred to collectively as “the wheel of life,” for living beings circle through them from rebirth to rebirth until they eventually learn the lessons of life and achieve enlightenment.
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Book reference: Gems of Wisdom from the seventh Dalai Lama by Glenn H. Mullin