Categorical judgments consist of a subject and a predicate. Predication occurs when the predicate is attributed to the subject. Predication cannot be made between two concepts in which there is no relation between them. Hence subject and predicate must be related at least one point. This common point ensures judgmental unity. This study examines the answer to the question ‘how the identity of the subject and predicate is ensured in a judgment or a proposition’. Therefore, our study aims to answer this question by dealing with Tajrid al-I'tiqad which was one of the most efficient works in mutaakhkhirun era and written by Nasir al-Din al-Tusi (d. 672/1273-74). Tusi remarked that subject and predication must be identical in a proposition. Yet according to him, this identity is not numerically identity but identity. Namely, the subject and predicate must be united at one (common) point. In our study, why exact identity cannot be supposed/counted is explained, the assumption of qualitatively identity is analyzed and concluded that the qualitatively identity theory ensures the required unity in a judgment.