因明入正理論疏 3.137

3.137

述曰：此指體也。即聲論師對薩婆多等立聲常、無礙，異喻如空。

The Commentary states: This points out its concrete form. That is, a Śābda theorist arguing against a Sarvāstivādin or the like establishes that sound is eternal and free from obstruction, using the heterogeneous example 'like space'.