The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut [1890] at sacred-texts.com
29. And on account of their difference of nature (the ideas of the waking state) are not like those of a dream.
We now apply ourselves to the refutation of the averment made by the Bauddha, that the ideas of posts, and so on, of which we are conscious in the waking state, may arise in the absence of external objects, just as the ideas of a dream, both being ideas alike.--The two sets of ideas, we maintain, cannot be treated on the same footing, on account of the difference of their character. They differ as follows.--The things of which we are conscious in a dream are negated by our waking consciousness. 'I wrongly thought that I had a meeting with a great man; no such meeting took place, but my mind was dulled by slumber, and so the
false idea arose.' In an analogous manner the things of which we are conscious when under the influence of a magic illusion, and the like, are negated by our ordinary consciousness. Those things, on the other hand, of which we are conscious in our waking state, such as posts and the like, are never negated in any state.--Moreover, the visions of a dream are acts of remembrance, while the visions of the waking state are acts of immediate consciousness; and the distinction between remembrance and immediate consciousness is directly cognised by every one as being founded on the absence or presence of the object. When, for instance, a man remembers his absent son, he does not directly perceive him, but merely wishes so to perceive him. As thus the distinction between the two states is evident to every one, it is impossible to formulate the inference that waking consciousness is false because it is mere consciousness, such as dreaming consciousness; for we certainly cannot allow would-be philosophers to deny the truth of what is directly evident to themselves. Just because they feel the absurdity of denying what is evident to themselves, and are consequently unable to demonstrate the baselessness of the ideas of the waking state from those ideas themselves, they attempt to demonstrate it from their having certain attributes in common with the ideas of the dreaming state. But if some attribute cannot belong to a thing on account of the latter's own nature, it cannot belong to it on account of the thing having certain attributes in common with some other thing. Fire, which is felt to be hot, cannot be demonstrated to be cold, on the ground of its having attributes in common with water. And the difference of nature between the waking and the sleeping state we have already shown.