By all counts, that which is knowable (the mind or ego) is merely unreal. It can never be called as the real Self because it is seen to be ephemeral. The experience (the claim) that ‘I know everything’ cannot be told in the case of ego etc., which are ephemeral.
Annotation
The Self is the non–negatable existence which is eternal. All that is objectified as ‘this’ is knowable and hence unreal. All experience is connected with the notion of self (‘I’) – such as ‘I see’, ‘I taste’, ‘I walk’ etc. The sense organs are limited to one mode of perception and all such mental modes are transitory and insentient. The delimited and impermanent ego cannot be called as the real Self. The mind cannot know itself, as in that case, it would be both the subject and object, which is a logical impossibility (Jagadguru).