心 is an anatomically observant drawing of an actual heart. Oracle bone forms preserve the outlines of left atrium, right atrium, and ventricle clearly — the ancient scribes had seen what they were drawing, perhaps from sacrificial animals. This single fact explains a deep cultural pattern: in CJK languages "mind" lives in the chest, not the head. Identical across 繁體 / 新字体 / 简体.

As a radical, 心 takes two positional forms. On the left of a compound it flattens into 忄 ("heart-side radical"); at the bottom it stays whole as 心. Both mark emotion or mental state: 思 (think — field + heart), 想 (imagine), 愛 (love), 念 (remember/recite), 怒 (anger — "slave heart"), 怕 (fear), 悲 (sad), 忘 (forget — "lost heart"), 忙 (busy — "lost mind"), 性 (nature — "heart of birth"). When you see 忄 or 心 inside a character, predict an emotion or mental verb.

Mandarin: xīn, level 1st tone. 心 covers heart (organ), mind, and center: 心脏 (xīnzàng, heart organ), 心情 (xīnqíng, mood), 中心 (zhōngxīn, center), 担心 (dānxīn, worry — "carry heart"), 关心 (guānxīn, care about), 小心 (xiǎoxīn, careful — "small heart"). The center-of-something meaning comes from the heart's location in the body.

Japanese: on-reading シン (shin) in 心配 (shinpai, worry), 中心 (chūshin, center), 安心 (anshin, peace of mind). The kun-reading こころ (kokoro) is one of the most beloved Japanese words — three syllables that mean heart, mind, and spirit indivisibly. 心 (kokoro) is also the title of Sōseki's 1914 novel, often considered the most important work of modern Japanese literature.

Memory aid: a four-stroke heart — the dot, the curl, two flames. The middle dot is the chamber.