The article is a suggestion to verify linguistic theories with the help of experimental methods applied in psycholinguistics. Conducted research concerned categorization of participles in Polish. Their place in the system is ambiguous when we try to classify them according to classical rules, based on mathematical logic. In the authoresses' experiment, they used the lexical decision task method, where verbal stimuli (nouns, verbs, adjectives, participles and gerunds were compared) differed from one another in the frequency of appearing in Polish, the length measured with the number of letters and their grammatical category. The response time was a measured variable. It turned out that there is a substantial difference in the response time between verbs and both nouns and adjectives, whereas there is no difference in the response time between participles and both verbs and adjectives. Additionally, the response time to participles indeed differs from the processing time of inflected forms of verbs. These results show that participia forms are derivatives situated among categories of verbs and adjectives.