فَالنَّفْيُ حَقِيقَةٌ مِنَ الْحَقَائِقِ
Negation, too, is a kind of reality or fact.
Someone who categorically asserts “no reality is established” has made a claim. If that claim is supposed to be true, the universal rejection of any truth faces a self-referential difficulty.
The reply addresses total denial. It does not establish the existence of every particular external object; it shows why a sweeping denial cannot straightforwardly rely on its own truth.
The fact that a negative judgment can be meaningful is distinct from proving that every external thing exists.