We can assume that any living creature must be built from atoms with the capability of forming highly organized systems. Despite previous comments about the subordination of material to arrangement, some materials are simply inappropriate. We cannot build a digital computer from wood, or even from metal using the crude shaped-metal technology of a century ago. Charles Babbage, mentioned in Chapter 5, understood and outlined the principles of a punched-card-controlled digital computer with a stored and modifiable program in 1833, but the technology of his time was inadequate to construct one.

Similarly, we cannot conceive of a living organism as being built mainly from ionic compounds. Nondirectional forces between ions do not permit the necessary degree of complexity. The main reason why we can claim "no life but carbon life" is that we see no other element in the periodic table that is capable of the extensive and varied molecular chemistry shown by carbon. There are good reasons why life is found only in a restricted range of size and temperature: This is the size range of macromolecules based on carbon, and of larger assemblies of such macromolecules; and this is the temperature range within which these compounds are relatively stable, yet reactions between compounds are reasonably fast.

We can summarize the arguments of this chapter by stating that life is the most exciting and challenging property exhibited by matter. It is a behavior pattern shown only by complex and well organized chemical systems.