The Brief Summary of Tàijíquán (太極約言) by Yáng Jiànhóu (楊健候)

太極約言

輕則靈，靈則動，動則變，變則化。

楊健候

"Light then agile, agile then mobile, mobile then variable, variable then neutralize."

From the book Tai Chi Secrets of the Yang Style (太極拳楊氏先哲秘要) by Master Yang Jwing Ming (楊俊敏).