Avicenna put the soul and its related issues at the forefront of his philosophical pursuits, and explained them in such a way that the later thinker could not ignore his views in their study of the topic. In his studies, he has dealt with the issues related to internal faculties of the soul in detail. One of these issues that he has introduced is the estimative faculty. He is considered as a pioneer in introducing this faculty among the other soul internal faculties. He has dealt with this faculty, its function for the soul, and its position in his works like Al Shifa and Al Isharat. The present study aimed to examine Avicenna’s views and evaluate their efficiency and characteristics. Based on the results of the study, conclusions are made about the principles of this faculty and its subordinated ones, and also distinctions are made between human beings and animals in terms of the faculty principles, their implications and products. Avicenna’s logical judgments in relation to the implications and the falsity and invalidity of a lot of judgment criteria among human beings are identified and the criteria sanctioned by Avicenna are revealed.