Complementarity vs. reduction as explanation of biological complexity.

The classical structure-function relation is presented as an example of an epistemological complementarity. Complementarity implies that a satisfactory explanation requires two modes of description, neither derivable from nor reducible to the other, as well as mutual incompatibility in a formal logical sense. Complementarity arose from the difficulty in describing conditional measurement processes in the language of causal microscopic laws of physics. Biological structure is associated with description in the language of physical laws, whereas biological function is associated with description of informational processes, i.e., measurement and control, that are complementary in the above sense. An explicit incompatibility between these two modes of description is that laws are expressed as rate-dependent equations, whereas informational processes are expressed as rate-independent nonintegrable constraints. The functional mode allows a simplification of structurally complex organization that is essential, not only for explanation, but for self-description at all levels of biological organization.