Undecidability without Arithmetization

In the present paper the well-known Gbdel's Church's argument concerning the undecidability of logic (of the first order functional calculus) is exhibited in a way which seems to be philosophically interesting. The natural numbers are not used. (Neither Chinese Theorem nor other specifically mathematical tricks are applied.) Only elementary logic and very simple set-theoretical constructions are put into the proof. Instead of the arithmetization I use the theory of concatenation (formalized by Alfred Tarski). This theory proves to be an appropriate tool. The decidability is defined directly as the property of graphical discernibility of formulas.