内心 nèixīn heart
heart; innermost being; (maths) incenter
How to remember 内心: 内 inside + 心 heart = heart.
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How it is used
- "Inner heart, innermost feelings" - always about emotion/thought, not the physical heart - 内心的想法 (nèixīn de xiǎngfǎ) one's inner thoughts, 内心深处 deep in one's heart.
- Contrast with 心里 (xīnlǐ), the everyday, more casual way to say "in one's heart/mind" - 内心 is more literary and used in writing or reflective speech.
- Never used for the physical organ - that's 心脏 (xīnzàng).