首楞嚴義疏注經 3.406

3.406

生滅非空難。虛空無為，為無生滅；今有生滅，不名無為。

This is the challenge that having arising and ceasing is not space. Empty space is unconditioned (asaṃskṛta) because it is without arising and ceasing; if it now has arising and ceasing, it cannot be called unconditioned.