Background and Purpose: Health and disease are among the most important basic concepts of medicine. However, defining their nature and limits, has faced serious challenges in medical philosophy. The purpose of this article is to explain Avicenna's point of view about health and disease and analyze the current definitions of these concepts based on Avicenna's view.

Materials and Methods: This study was performed by a deductive approach in grounded theory method. Initially, the concepts of health and disease were studied in conventional medicine and books written by Avicenna. These concepts were then used as initial propositions in a deductive argument to create operational propositions.

Results: Avicenna considers health as an accident and one of the qualities of the soul. He defines it as a habit or disposition that is the cause of normal functions in its subject. He also describes disease as a non-healthy condition and considers the assumption of a third state between the two to be unnecessary. The World Health Organization (WHO) defines health to be complete physical, mental, and social well-being, not merely the absence of disease. Such a definition requires the assumption of a third state between health and disease. According to Avicenna, this means to consider both concepts as existential issues; and eventually excludes a large number of dysfunctions -which are not categorized as known diseases or disease symptoms- from the physician's attention. Avicenna accounts disease as a matter of non-existence and considers any level of dysfunction in any function as a level of disease.

Conclusion: Based on the Avicennian theoretical framework, limiting the health concept boundaries to an optimal state in various dimensions is the strength point of the WHO definition of health; while its weakness is the lack of elegance to cover all subnormal conditions in all functions, under a broader concept of disease.