How we understand our surroundings is an age old question. The general understanding is that it is through concepts, categories, and relations that we make sense of our world. This study presents a result of a dissertation that attempted to investigate the nexus between human cognition of concepts/categories and the knowledge structure in the unified medical language system (UMLS). In contrast to the classical account that views category as structures organized in terms of similarity, this study takes a new approach that explains categories as structures organized around theories. The findings reveal that humans largely use simplified rules, definition type descriptions, and naïve explanations as bases for categorization judgment (coherence criteria). These explanations research participants used to characterize their category formation, after a careful text analysis, created meaningful and coherent interrelationships when plotted in a visual link chart. The performance of the cognitive task also showed a remarkable difference between those with subject background and those without. Using the least restrictive notion of ‘distance’ as a function of relatedness, this study also showed a significant correspondence between human conceptual map (based on the cognitive performance) and the knowledge structure in the UMLS, as shown by the goodness-of-fit coefficient.