ABSTRACT. The author presents a theory which helps to solve the popular skeptic puzzle: 1. If you do not know that you are not a brain in a vat, then you do not know anything, 2. You do not know that you are not a brain in a vat. Therefore: 3. You do not know anything. He claims that the concepts which we use to attribute propositional attitudes are notoriously ambiguous. Cognitive (belief, certainty, truth, justification, knowledge) and perceptual (impression and perception) concepts are analyzed. The author divides them into two divisions: subjective (belief (subjective truth, subjective knowledge), certainty (subjective justification), impression (subjective perception)) and objective (objective truth, objective justification, objective knowledge, objective perception). The basic claim is that only an external attributor may use the objective cognitive concepts toward the subject. The solution of the puzzle is that the second premise (2) is always false no matter if you consider it in the subjective or objective meaning and if the attributor believes or not that you are a brain in a vat. Kant’s theory of understanding, Wittgenstein’s Tractatus and Tarski’s theory of truth are mentioned too. pp. 96–120

Keywords: knowledge; skepticism; belief; certainty; truth; justification; impression; perception; objective; subjective