The results f an experiment during which pupils of the Elementary and Secondary Schools w ere administered pairs of letters half o f which contained besides various subsidiary letters a common character Z, the other half the common character C, confirm the fact that younger pupils analyse complex stimuli less easily than older ones. Those individuals who succeded to differentiate stimuli with different main letters, never responded to pairs of subsidiary ones. As the main letters in the presented stimuli recurred altogether fifteen times and the subsidiary ones three times, the differentiation of the main and subsidiary signs within reinforced as well as unreinforced pairs of letters can be explained by differences in excitation, in accordance with Sechenov’s and Forster’s interpretation. On the other hand, the differentiation of the two main characters, which recurred in the experiment with the same frequency, was arrived at by the pupils by trial and error on the basis of their different signalling significance, according to Pavlov's supposition. These findings, indicating a possibility of reconciling the two apparently contradictory views on the formation of ideas, are of considerable importance also for didactics.