The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com
30. The existence (of mental impressions) is not possible (on the Bauddha view, on account of the absence of perception (of external things).
We now proceed to that theory of yours, according to which the variety of ideas can be explained from the
variety of mental impressions, without any reference to external things, and remark that on your doctrine the existence of mental impressions is impossible, as you do not admit the perception of external things. For the variety of mental impressions is caused altogether by the variety of the things perceived. How, indeed, could various impressions originate if no external things were perceived? The hypothesis of a beginningless series of mental impressions would lead only to a baseless regressus ad infinitum, sublative of the entire phenomenal world, and would in no way establish your position.--The same argument, i.e. the one founded on the impossibility of mental impressions which are not caused by external things, refutes also the positive and negative judgments, on the ground of which the denier of an external world above attempted to show that ideas are caused by mental impressions, not by external things. We rather have on our side a positive and a negative judgment whereby to establish our doctrine of the existence of external things, viz. 'the perception of external things is admitted to take place also without mental impressions,' and 'mental impressions are not admitted to originate independently of the perception of external things.'--Moreover, an impression is a kind of modification, and modifications cannot, as experience teaches, take place unless there is some substratum which is modified. But, according to your doctrine, such a substratum of impressions does not exist, since you say that it cannot be cognised through any means of knowledge.