십팔공 (十八空)의 하나로, 육근 (六根) (눈, 귀, 코, 혀, 몸, 뜻)이 공함을 말하며, 이들은 모두 실제의 자성이 없으므로 내공이라 말함
Internal non-substantiality or emptiness: One of the eighteen aspects of non-substantiality or void. Emptiness of six roots or sense faculties (the eye, ear, nose, tongue, body, and mind). They are called empty because they have no self-nature, essential substance, or self-existence. Cf. (Sippalgong) Eighteen kinds of emptiness.