The paper defends the thesis that the only possible answer to the global skepticism is the so called ontological proof: only God's truthfulness and His bonitas can guarantee the rejection of such skeptical hypothesis as the one of R. Decscartes' stating that we are deceived by some evil demon or H. Putnam's claim that we are brains in vats. The author proposes an interpretation of the ontological proof in the spirit of I. Kant's considerations from his 'Der einzig mögliche Beweisgrund zu einer Demonstration des Daseins Gottes' (1762) where Kant uses the concepts of necessity and possibility in order to proof God's existence. Existence is a perfection and hence it must be one of the attributes that form the idea of God. Also truthfulness and bonitas must be treated as belonging to the idea of a perfect being. The truth condition for the sentence 'Perfect necessary being exists' is the real existence of a perfect necessary being but at same the time the truth condition for this sentence is its condition of possibility. This sentence could not be possible, if the object it refers to had not existed, e.g. if the perfect necessary being had not existed. Because this sentence is something real it must be also possible and from this follows that the perfect necessary being exists. Analogically, every attempt to suppose that it is possible that perfect necessary being does not exist presupposes real existence of perfect necessary being because nothing could be possible, if something were not necessary. 'Perfect necessary being exists' is the only sentence where the truth condition and condition of possibility coincide in this way. God's truthfulness and His bonitas guarantee that we are not deceived by some evil demon or that we are not brains in vat.