pledge, nature

質 is composed of 斦 (chopping board) and 貝 (shell). 斦 carries the meaning of mutual cancellation or offsetting, while 貝 refers to wealth or valuables. Therefore, the original meaning of 質 is something given as a pledge or collateral. Unlike a verbal promise, a pledge is a tangible object; from this, 質 extended to the fundamental sense of nature or essence.