The need for a new biological model in geratology.

Disease classes have hitherto been based on anatomical pathology and structural lesions belonging to an earlier ecological medical model involving classification. The diseases of ageing now coming to dominate clinical practice evolve across the clinical threshold in middle age parallel to changes occurring in the internal environment. The concept 'multiple pathology' used to describe the plural features of biological changes now requires new intellectual tools by which to understand overlap phenomena. Boolean algebra and Set Theory are proposed as the relevant enabling concepts, and their application to modern clinical practice is discussed.