125 systems often injure us by treating various parts of ourselves as though they too, were infectious invaders. This blindness leads to conditions such as diabetes, multiple sclerosis, rheu- matoid arthritis, and many others. We are also subject to injuries that our bodies cannot repair. Namely, accidents, dietary imbalances, chemical poisons, heat, radiation, and sundry other influences can deform or chemi- cally alter the molecules inside our cells so that they are unable to function. Some of these errors get corrected by replacing defective molecules. However, when the replacement rate is too slow, errors accumulate. For example, when the proteins of the eyes’ lenses lose their elasticity, we lose our ability to focus and need bifocal spectacles. BIOLOGICAL WEARING-OUT Most likely, eventual senescence is inevitable in all biologi- cal organisms. As we learn more about our genes and cellular biochemistry, we will hopefully be able to correct, or at least postpone many conditions that still plague our later years. However, even if we found cures for each specific disease, we would still have to deal with the general problem of ‘wearing out’. The normal function of every cell involves thousands of chemical processes, each of which sometimes makes random mistakes. Our bodies use many kinds of correction techniques, each triggered by a specific type of mistake.  However, those random errors happen in so many different ways that any low- level scheme to correct them would be difficult indeed. The problem is that our genetic systems were not designed for  very  long-term  maintenance.  The  relationship  between genes and cells is exceedingly indirect. To repair defects on larger scales, a body would need some sort of catalogue that specified where the various types of cells should be located Essays on Infinite Lifespans   Marvin L. Minsky, Ph.D.