首楞嚴義疏注經 3.426

3.426

標徵也。隨出土之多少，則見空之淺深也。虛空與色二俱是假，互相因有，體不離色，今此推徵令知虛妄。

This frames the investigation. Following the quantity of soil removed, one sees the depth of the space. Space and form are both unreal, existing in mutual dependence; its substance is not separate from form. Here the interrogation causes one to realize their falsity.