請觀音經疏闡義鈔 2.24

2.24

「十八空」者，空唯是一，破十八有，名十八空，謂內空、外空、內外空乃至無法有法空也。

Regarding “Eighteen emptinesses”: emptiness is only one, but destroying eighteen kinds of existence is called the eighteen emptinesses—namely, internal emptiness, external emptiness, internal-and-external emptiness, down to the emptiness of non-existence and existence.