He also examined the concept of 'essentially undecidable' introduced by Tarski, and answered an important open question by constructing a theory with a finite number of axioms that is essentially undecidable.
Siya din napagmasdan ang konsepto ng 'mahalagang undecidable' ipinakilala sa pamamagitan ng Tarski, at sumagot ng isang mahalagang bukas na katanungan sa pamamagitan ng constructing isang theory na may hangganan bilang ng mga axioms na ay mahalagang undecidable.