大方廣佛華嚴經隨疏演義鈔 59.13

59.13

疏「響可喻言聲」者，此喻亦是遮救無知，謂無情之物，動與不動故無知覺。眾生有心，安無知覺？故引響喻。因心發聲，扣谷發響。響無知覺，情發亦然。

The Commentary states: "An echo can illustrate the spoken sound": this analogy also serves to prevent defending [a soul's] absence of cognition, claiming that insentient things, whether moving or unmoving, are therefore devoid of cognition and sentience, but since sentient beings possess mind, how could they lack cognition and sentience? Therefore, the analogy of an echo is cited. Sound is produced due to mind; striking a valley produces an echo. An echo has no cognition or sentience, and what is produced from consciousness is likewise so.