According to Unani practitioners, the failure of the "Quwwat-e-Mudabbira-e-Badan", or the body's ability to maintain its own health, may lead to derangement of the normal equilibrium of the body's "akhlat" (humors). Abnormal humors are believed to lead to pathological changes in the tissues at the affected site, creating the clinical manifestations of illness. The theory postulates the presence of blood, phlegm, yellow bile and black bile in the human body. Each person's unique mixture of these substances determines his "mizaj" (temperament). A predominance of blood gives a sanguine temperament; a predominance of phlegm makes one phlegmatic; yellow bile, bilious (or choleric); and black bile, melancholic.