The Scientist has a wonderful article about complimentarity in biology. Complimentarity is the application of two or more different theoretical approaches to a single problem:

"Light and Life" is perhaps best known for its focus on Bohr's concept of complementarity. According to this concept, some natural phenomena can only be completely understood by combining two or more experimental approaches that cannot be simultaneously implemented. More generally, complementarity asserts that apparently incompatible ideas or perspectives can both be necessary to achieve a fuller understanding of an entity or process.

Bohr's original inspiration for the concept of complementarity was that ultimate source of illumination, light. He was impressed by the fact that some phenomena involving light (e.g. diffraction patterns) were best explained by regarding light as waves. However, other phenomena involving light (e.g., the photoelectric effect) were best explained by regarding light as particles. Thus, it may not be justified to claim unqualifiedly that light is waves or that light is particles. Instead, light may be better understood as behaving as if it were particles or waves, depending on the precise methods of detection or measurement.

In "Light and Life," delivered in Copenhagen to the International Congress on Light Therapy, Bohr specifically proposed that there is a complementarity relevant to biology, as well, arguing that it may not prove possible to fully explain living processes in physical terms. This notion, depending on precisely how it is interpreted, is generally dismissed, and appropriately so to the extent that it might be confused with nebulous concepts of a uniquely life-associated "vital force" (to which Bohr did not subscribe). But even if Bohr was wrong in his literal thesis, he may nevertheless have been onto a biologically useful insight.

Bohr's student in theoretical physics, Max Delbruck, eventually left physics for biology and became the strongest exponent of the relevance of complementarity to biology. He called attention to the fact that, as he put it, molecular structure and integrated biological function may not be compatible observables. Structure determination typically requires molecular homogeneity, while determination of integrated biological function necessarily requires the molecular heterogeneity of the cellular or organismal environment. In other words, to understand molecular structure you need chemical simplicity, while insights into function (at least in one sense) require the full chemical complexity of the whole biological system.

Read the whole thing.