In the human thinking, a relevant aspect is the way to extract new knowledge, combining the actual information, to proceed from established rules. The deductive judgement supposes this operational aspect of thinking with implications in human decision. The informational combinative way (syllogistical test) is related with the individual ability to activate informational units from long-term memory to operational memory and work with them in a task (operational memory test) and with the general developmental level of aptitude (general aptitude test). The goal of this study hinted:
- the relation between deductive thinking, operational memory and general abilities;
- the main errors intervened when the subject makes an inference. We found that:
a. the level of general ability is the foundation of syllogistical inference;
b. the syllogistical error depends to the logically shape or to content of premises;
c. the subject's tendency is to hardly infer conclusion that, logically valid, runs counter to their previous knowledge;
d. the cognitive scheme constitutes a source of syllogistical errors.
KEYWORDS: syllogistical reasoning, working memory, storage, errors.