成唯識論述記 2.25

2.25

述曰：此正難也，承光、發影東西不同，故知極微定有方分，如日照一柱，其中極微無方分者，應日照東處，西邊有光，無方分故，應無所隔。汝之極微應有方分；即和合色故；如和合色，東處非西，明有方分。

The Commentary states: This is the direct challenge: catching light and casting shadows differ between east and west, hence we know atoms definitely have spatial parts. If, when the sun illuminates a pillar, the atoms in it lacked spatial parts, then when the sun illuminated the eastern side, the western side would also have light; because lacking spatial parts, there should be no obstruction. 'Your atoms should have spatial parts; because they are composite matter; like composite matter.' The eastern side is not the west, clearly proving they have spatial parts.