内心(nèi xīn) - inner heart, innermost feelings in Mandarin

内心 (nèi xīn) is the private inner self: what a person actually feels underneath whatever they are showing. It is the word behind most English sentences that begin deep down or in his heart of hearts.

In practice it works adverbially far more often than as a subject. 他内心很矛盾 means he is torn inside, with 内心 sitting between subject and comment. 内心深处 intensifies it to the very bottom of one's heart.

Two traps. It is never the physical organ, which is 心脏, so a heart attack has nothing to do with 内心. And it is not interchangeable with 心里: 心里 is the everyday spoken container for passing thoughts and feelings, while 内心 is more literary and points at what a person truly is.