The author deals with the operational core of logic, i.e. its diverse procedures of inference, in order to show that logically false inferences may in fact be right because - in contrast to logical rationality - they actually enlarge our knowledge of the world. This does not only mean that logically true inferences say nothing about the world, but also that all our inferences are invented hypotheses the adequacy of which cannot be proved within logic but only pragmatically. In conclusion the author demonstrates, through the relationship between rule-following and rationality, that it is most irrational to want to exclude the irrational: it may, at times, be most rational to think and infer irrationally. Focussing on the operational aspects of knowing as inferring does away with the hiatus between logic and life, cognition and the world (reality) - or whatever other dualism one wants to invoke -: knowing means inferring, inferring means rule-governed interpreting, interpreting is a constructive, synthetic act, and a construction that proves adequate (viable) in the “world of experience”, in life, in the praxis of living, is, to the constructivist mind, knowledge. It is the practice of living which provides the orienting standards for constructivist thinking and its judgments of viability. The question of truth is replaced by the question of viability, and viability depends on the (right) kind of experiential fit. (Hans Rudi Fischer)