俱舍論疏 8.177

8.177

論「然諸因緣至難可思議」，此釋像雖實無，因緣令見。難思議者，不測所以然也。

On the passage in the śāstra from "However, various causes and conditions" to "are difficult to conceive": This explains that although reflections do not exist in reality, causes and conditions make them visible. That they are "difficult to conceive" means that the reason why it is so cannot be fathomed.