The Buddha explains to Venerable Rādha that a living being is defined by desire, passion, delight, and craving in form, feeling, perception, formations, and consciousness.
Satta sutta - Living Beings
At Sāvatthi.
Seated to one side, Venerable Rādha addressed the Blessed One:
"Venerable sir, it is said 'being, being.' To what extent, venerable sir, is one called a being?"
The Blessed One replied, "Rādha, in form, wherever there is desire, passion, delight, and craving, there one is a being, there one is bound, therefore, one is called a being. In feeling, wherever there is desire, passion, delight, and craving, there one is a being, there one is bound, therefore, one is called a being. In perception, wherever there is desire, passion, delight, and craving, there one is a being, there one is bound, therefore, one is called a being. In formations, wherever there is desire, passion, delight, and craving, there one is a being, there one is bound, therefore, one is called a being. In consciousness, wherever there is desire, passion, delight, and craving, there one is a being, there one is bound, therefore, one is called a being.
Just as, Rādha, young boys or girls play with sandcastles: as long as they possess passion, desire, affection, thirst, fervor, and craving for those sandcastles, they cling to them, engage playfully, hold them dear, and consider them as their own. But when, Rādha, the young boys or girls lose their passion, desire, affection, thirst, fervor, and craving towards those sandcastles, then they scatter them with their hands and feet, demolish and destroy them, rendering them unplayable."
In just this way, Rādha, you too should scatter, demolish, and destroy form, and make it unplayable by practicing for the wearing away of craving. You should scatter, demolish, and destroy feeling, and make it unplayable by practicing for the wearing away of craving. You should scatter, demolish, and destroy perception, and make it unplayable by practicing for the wearing away of craving. You should scatter, demolish, and destroy volitional formations, and make it unplayable by practicing for the wearing away of craving. You should scatter, demolish, and destroy consciousness, and make it unplayable by practicing for the wearing away of craving. Indeed, Rādha, the complete wearing away of craving is Nibbāna."