Not a symbol but an actual heart, drawn from looking: the chambers, the vessels leaving the top. The ancient Chinese located thinking here, in the chest — so 心 means both "heart" and "mind", feeling and thought in a single drawing.
A piece of culture
The thinking heart Classical Chinese philosophy never split head from heart: the 心 was where you thought AND felt, which is why philosophers translate it as "heart-mind". Look inside characters and you find it everywhere feelings live: to think (想), to forget (忘), to be busy (忙 — a heart dying of overwork!), and love itself. The traditional character for love, 愛, carries a 心 in its very center — which is why some say the simplified form 爱, which dropped it, is love without a heart.