俱舍論疏 29.159

29.159

論「若彼遂謂至便壞自宗」，破轉計也。若轉計云：我非識所緣，應非所識。若非所識，即非所知。汝既不知，如何立有？若不立有，便壞自宗。

The treatise says: "If they should then claim... they would undermine their own doctrine." This refutes an alternative assertion. If they turn and assert: "The self is not an object of consciousness, so it should not be cognized," then if it is not cognized, it is not an object of knowledge. Since you do not know it, how can you establish that it exists? If you do not establish that it exists, you undermine your own doctrine.