Several parts combine into one character.

Compound ideograph: originally 囟 (the fontanel / soft spot at top of skull, looking like a small box with internal lines) above 心 (heart). The encoded definition: "what the head and the heart do together" = thinking. East Asian thought refused to separate reason from emotion — both organs participate in cognition. Over centuries the top component simplified to 田 (which now resembles "field" but is really the abstracted skull-fontanel). Identical across 繁體 / 新字体 / 简体.

This is one of the most philosophically loaded characters in the script. Western philosophy from Descartes onward separated reason (mind) from emotion (heart); 思 explicitly refuses that separation. To "think" in CJK languages is to deploy both mental and emotional faculties — the character stamps that worldview onto every act of cognition.

Mandarin: sī, level 1st tone. 思考 (sīkǎo, to think / consider — formal), 思想 (sīxiǎng, thought / ideology), 意思 (yìsi, meaning / "what something signifies" — neutral final tone, EXTREMELY high frequency: 这是什么意思? "What does this mean?"), 相思 (xiāngsī, mutual longing / lovesickness), 反思 (fǎnsī, reflection). 意思 is one of the first essential Mandarin words for any learner.

Japanese: on-reading シ (shi) for compounds — 思想 (shisō, thought), 意思 (ishi, intention / will), 思考 (shikō, thinking / cognition). Kun-reading おも.う (omo.u, to think / feel) is among the highest-frequency Japanese verbs — 思う sits at the top of the daily verb list alongside する, ある, いる. Compounds: 思い出 (omoide, memory — "the thought that comes out"), 思いやり (omoiyari, consideration / empathy), 思いつく (omoitsuku, to come up with an idea).

The distinction between 思う (omou, "to think / feel" — lighter, internal) and 考える (kangaeru, "to think / consider" — heavier, more deliberate) is one of the foundational verb pairs in Japanese.

Memory aid: skull (田) over heart (心) — thinking is mind AND emotion together.