It was systematized by Yi Je-ma in his book Dongyi Suse Bowon: Longevity and Life Preservation in Eastern Medicine (동의수세보원, 東醫壽世保元) in 1894.
[1] It divides people into four body types based on their biopsychosocial traits.
The classification was derived from the five body types of Traditional Chinese medicine described in an ancient Chinese medical book Lingshu Jing of Huangdi Neijing.
[2] Sasang typology divides people into four types based on their biopsychosocial traits with a combination of yin/yang and greater/lesser: tae-yang ( 태양, 太陽) or "greater yang"; so-yang (소양, 小陽) or "lesser yang"; tae-eum (태음, 太陰) or "greater yin"; and so-eum (소음, 小陰) or "lesser yin".
Each type consists of a classification of pathology, medicine and hygiene depending on personality, psychological status and organ functionality.