大乘起信論義疏 2.4

2.4

問曰：此智能顯真，故經中說為了因也。何故今說為智障乎？多義如真，故復名障。如藥治病，若藥不去，藥復成患。此亦如是。

Question: This wisdom can reveal the real; thus the sūtra explains it as an illuminating cause (kāraka-hetu). Why is it now spoken of as an obstruction to wisdom? [Answer:] Although it conforms to the real, it is nonetheless called an obstruction. Just like medicine curing a disease: if the medicine is not cleared away, the medicine again becomes an affliction. This is also like that.