think by oneself 独立思考think for oneself 为自己着想 比如Jack is old enough to think for himself杰克年纪不小了，可以自己拿主意了这里的自己拿主意的意思就是为自己考虑，自己做决定的意思
think by oneself某人自己思考think for oneself 为某人考虑
由某人自己去想 ,为某人想想
