Читать книгу Astrobiology - Charles S. Cockell - Страница 27

Thus far, we have seen that it is difficult to define life. An obvious problem seems to be our inability to uniquely list or state the characteristics that sharply separate it from non-life. One answer to the problem of the definition of life is that life is simply a human word, an artificial definition created by us. It is what philosophers would call “a non-natural kind,” as opposed to a “natural kind.”

The term “natural kind” applies to a substance such as gold, whose characteristics can be exactly defined in terms of its physical properties (Figure 2.13). We can state the molecular mass, melting point, and a range of other definitive physical properties of gold that allow for an exact definition of what it is. The reason that it is a natural kind is that the word gold is used to denote a very specific type of atom. It is limited by something that can be described in very specific physical terms.

Figure 2.13 Gold can be uniquely defined using physical characteristics such as its melting temperature and atomic mass number. Is life in the same category of object?

A good example of a “non-natural kind” is a “chair” (Figure 2.14). If we define it as “something we can sit on,” then does that make my coffee table a chair? You might reply that a coffee table is not a chair because it has no vertical back on it. I might then show you a stool and claim that it too can therefore not be a chair and is more like a table. Thus, we launch into an endless circular discussion about what a chair is. The conversation is rather pointless because ultimately a chair is simply what we define it to be. If that includes coffee tables then so be it.

Figure 2.14 Perhaps life is more like a chair? Something whose definition depends more on human categorizations, rather than any fundamental physically derivable difference with other materials.

Similarly, maybe life is just a definition that encompasses an interesting segment of all organic chemistry that happens to do certain things, such as reproducing and growing. If we want to include viruses, then so be it; if not, then so be it. Perhaps the crucial point is that we all attempt to agree on a definition that we are going to use that includes all chemistry we are going to call “life.”

The notion that life might be a human definition and not rooted in physically separable characteristics might make some intuitive sense. Organic chemistry runs along a continuum from simple gases, such as methane, through to more complex organic molecules, through molecules that can replicate, such as certain ribonucleic acids, past molecules wrapped in protein that can replicate if given the correct environment (viruses) through to cellular materials and on to single-celled organisms and multicellular organisms. Perhaps life is a division we place somewhere along this continuum that will inevitably have certain materials whose inclusion on either side of the line will always be a matter of debate.