Like the earth, the arahat is patient and is not provoked to respond in anger; like the door-post he is firm; he is unperturbed by the ups and downs of life; he is serene and pure like a lake free from mud. For such an arahat there will be no more rebirth. - Dhammapada verse 95
An Arahat is one that has realized that the inside and outside are the same. Or one that has realized the emptiness of self. In AYP terms, it would be "unity".