The transformation of a meaning of prostota This article is dedicated to the evolution of one of prostota lexeme meaning (and prostoć, prostość as well) – its human mental sphere significance. The analysis (definitions and examples) expands over the Polish language dictionary since its beginning till modernity. There were two main meanings of a lexeme prostota from the Old Polish till the end of the XVII century – positively characterised straight meaning regarding morality, directly linked to religious attitude and the second one, referring to mental and intelectual ability of individual, negatively regarding its undernormative features. It transformed during the XVIII century – the disredarding meaning disappeared, the first, religious aspect expanded over morality, customs, the way of living, simultanously drifting away from its religious significance. Throught the history of the prostota lexeme there was a short period regarding to a lack of culture and misbehaviour but dissapeared quickly with uprising of a prostactwo lexeme. Since the XIX century the dominating feature of prostota lexeme is being natural in a positive way. Besides the modifications of the lexeme and the transformation of its meanings prostota was mostly referring to positive values, which are the only meanings now.