With science advancing, the very definition of life has become a hot topic for biologists and philosophers. When high-powered microscopes confirmed that viruses are essentially just proteins storing genetic information but lack all other elements of living cells, there was substantial debate. Are viruses alive at all, or are they just nonliving matter? In his essay, Chance and Necessity: Essay on the Natural Philosophy of Modern Biology, Nobel Prize winner and French biologist Jacques Monod discussed teleonomy, or goal directedness, and stated that the common denominator of life is that it follows a clearly identifiable purpose. In viruses, this purpose is reproduction. They enter the cells of a host organism and force them to produce copies of themselves—more viruses. In more complex life forms, the underlying purpose of reproduction is covered with other layers that can still be traced back to reproduction. Take the playful behavior of young animals as an example. In playing, they mimic the actions of adults and thereby practice skills that will be essential for their survival. Teleonomy is closely related to concepts of complexity theory and self-organizing systems. Some philosophers of biology resist the term, whereas others endorse it.