This is Son/Zon, 'Exist/Know/Think'. I confess I like this one for meaning, the implied correlation between thinking and existance; the etymology is not so interesting.
The blocky square in the background is a variation of a character meaning 'talent' and denotes a dam. Substantial and solid, firm existance. The squiggle inside is the character for 'child', but it was actually added for phonetic reasons - the words for 'child' and 'stack' (of stones) are similar. It reinforces substantial/real/solid/stubborn.
It is not clear how, in Japanese, this character came to mean 'know' and 'think'. At least not to any of the sources I've got. :)