Literature Reference: 外物不可必，中怀须自空。—— 《闻庾七左降因咏所怀》 External things cannot be relied upon, the heart must be empty of itself. Source from 'Reflections on the Demotion of Yu Qi'.
自: Nature; the true self.
空: The heavens, the expanse.
Meaning: To be empty within means to free the heart from external burdens, maintaining inner peace and freedom.