The processes of sense generation and sign generation are obviously not isomorphic. We consider Cogito taking into account this fact. Decartes supposes the сogito-style sentences (Cogito, Dubito, Ego sum res cogitans, etc.) are unconditional and unfalsifiable. We argue that Descartes, while considering the sign version of Cogito, uses some hidden premises in spite of the well-known declaration there are none. They are:1) there is a time interval Δt over which I cannot be wrong on the matter of what happened during Δt (infallible time); 2) there is a time interval Δt upon which I cannot be wrong (say, if an event happened quickly from I-viewpoint then if measured in some standard units, one will qualify it as “quick” as well). We investigate the advantage the evil demon can take from that. This is the sign manipulation we propose. Cogito, as a sign object, а) has to have a sense-data component and b) can be built only through time by consecutive generation of one elementary sign after another. Then an (elementary) sign manipulation is the following: (1) I is [not am] building the sign si (or some part of it, which is not a sign any longer), (2) I thinks wrongly that the previous signs just built are s1 , ... , si-1 (the evil demon’s work), (3) I is [not am] going to built signs si+1 , ... , sn. For example, one thinks of self as thinking Cogito, while in fact the real event is the building of t (the part cogi, which is prior to t, is in the past and as such can be manipulatively substituted instead of what really happened in the life of I before building the sign t - whatever it is; probably, nothing; probably, I didn’t exist before generating t). We investigate the source which makes the manipulation possible and the gravity of the destruction of such sort. The crucial reason for the possibility of such misconception turns out to be the sense-data component in any linguistic (sign) constructions.