The principle of substance stability - the feedback principle - is applicable to all biological systems. It boils down for different temporal hierarchies to the following: during the formation (self-assembly) of the most thermodynamically stable structures at the highest hierarchical level (j), e.g., the supramolecular level, in accordance with the second law, Nature spontaneously uses predominantly the ( available for the given local part of the biological system) least thermodynamically stable structures belonging to a lower level, for example, the molecular level (j-1). The principle can be also applied to understructure hierarchical levels of any temporal hierarchy. It helps in understanding the causes of cancer formation and the origin of some other diseases.