Most of the complex carbon compounds that exist on Earth were produced by such organized, living chemical systems.The free energy necessary to drive these nonspontaneous reactions "uphill" is obtained from other carbon-based molecules in the environment. Most of this energy results from the destruction of carbon-containing molecules and the combination of their atoms with oxygen. These organized entities maintain low entropy within their own boundaries by creating even more entropy outside.

The ultimate source of free energy to drive all these chemical systems is the sun. Living chemical systems use a minute fraction of the solar radiation (less than one part in a billion is intercepted by the Earth) to maintain a temporary state of high order in localized regions of space on the surfaces of our planet. The activities necessary for these systems to maintain themselves, to obtain free energy, and to avoid destruction, all have the appearance of being purposeful. Some of these localized chemical units seem more purposeful and more adaptive to changing conditions than others. No matter what the chemistry of our hypothetical outside observer might be, he would undoubtedly recognize such carbon-based chemical systems as being alive.

To turn matters around and observe the observer, could such an alien visitor exist with a chemistry based on other than carbon atoms? The answer probably is no. We cannot give a meaningful answer to this question until we know more about the behaviour of carbon compounds and of living systems.