They deny any non-entity ulterior to entity. Non-entity being cognisable by the sixth instrument of knowledge (_anupalabdhi_), and knowledge being always an object of inference, the absence of knowledge cannot be an object of perception. If, again, any one who maintains non-entity to be perceptible should employ the above argument (from the perceptions, I am ignorant, I know not myself, and other things); it may be replied: "Is there, or is there not, in the consciousness, I am ignorant, an apprehension of self as characterised by an absence, and of knowledge as the thing absent or non-existent? If there is such apprehension, consciousness of the absence of knowledge will be impossible, as involving a contradiction. If there is not, consciousness of the absence of knowledge, which consciousness presupposes a knowledge of the subject and of the thing absent, will not readily become possible." Inasmuch (the Śáṅkaras continue) as the foregoing difficulties do not occur if ignorance (or illusion) be entitative, this consciousness (I am ignorant, I know not myself, and other things) must be admitted to be conversant about an entitative ignorance. All this (the Rámánuja replies) is about as profitable as it would be for a ruminant animal to ruminate upon ether; for an entitative ignorance is not more supposable than an absence of knowledge. For (we would ask), is any self-conscious principle presented as an object and as a subject (of ignorance) as distinct from cognition? If it is presented, how, since ignorance of a thing is terminable by knowledge of its essence, can the ignorance continue? If none such is presented, how can we be conscious of an ignorance which has no subject and no object? If you say: A pure manifestation of the spiritual essence is revealed only by the cognition opposed to ignorance (or illusion), and thus there is no absurdity in the consciousness of ignorance accompanied with a consciousness of its subject and object; then we rejoin:--Unfortunately for you, this (consciousness of subject) must arise equally in the absence of knowledge (for such we define illusion to be), notwithstanding your assertion to the contrary. It must, therefore, be acknowledged that the cognition, I am ignorant, I know not myself and other things, is conversant about an absence of cognition allowed by us both.