The author sets out to present the most basic model of economic rationality (omitting uncertainty, strategic interaction and information shortage), along with its assumptions, the statements derivable from those assumptions, and the interpretations of these. This model captures the behaviour of economic decision-makers from the side of declared preference. After presenting the work and findings of Marcel K. Richter, the author examines what rationality precisely means, what can be understood by it and what not, for numerous errors and misunderstandings about the question of rationality are prone to appear in professional literature.