Well over time I continued to discuss the topic with friends, and a few weeks ago I rewrote the argument. I am trying to say the exact same thing I said before, but my goal is to say it in a more formal and exact way. Let me know what you think. What follows is lightly adapted from an email I sent my friends at the beginning of this month.
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My claim, which I shall undertake to prove, is that the normal Darwinian picture of blind evolution logically entails the existence of natural purposes at the level of the individual organism and at the level of the species. More precisely, I propose to show that it is possible to define the word "purpose" and the phrase "natural way of life" in ways that are (1) consistent with Darwinian theory and also (2) intuitively plausible.
Please note that throughout this argument I will use the language of Darwinian theory as it has trickled down to the ordinary, university-educated non-biologist, on the assumption that it is all just a fact. Partly this is because my layman's understanding is that neo-Darwinian theory is pretty well established by analysis of the available data. But in addition, in this case the whole point is to show that Darwinian theory itself logically requires the results I have described. So it only makes sense to start out by assuming it. At the same time, I am neither a specialist in neo-Darwinian theory nor even a biologist. So it is certain that there are specialized features of the theory that I do not know. I am operating with only a layman's knowledge.
Now, to restate formally what I intend to show:
Theorem: Assuming that there are no purposes in the heavens and that life evolved in a Darwinian way by random chance, it is nonetheless possible to define the word "purpose" in three ways (corresponding to three different levels of analysis) such that in each case the word describes something which (1) must exist by Darwinian theory and (2) plausibly corresponds to our normal use of the word. These three different uses shall be distinguished as purpose1 (when discussing ontogenesis), purpose2 (when discussing the life of the individual organism), and purpose3 (when discussing the evolution of particular organs or features). Moreover it is possible to define the phrase "natural way of life" such that it describes something which (1) must exist by Darwinian theory and (2) plausibly corresponds to our normal use of the phrase.
Part 1: Ontogenesis is the growth of any organism from a fertilized egg into an adult. In this context, the purpose1 of any preliminary structure in the embryo is defined to be the organ that it will grow into by the time of maturity. So the purpose1 of these cells here is to grow into the lungs, while the purpose of those cells there is to grow into the stomach. Philosophically, this is not a very interesting usage. But please note that any normal understanding of Darwinian biology guarantees the existence of purpose1 for any species.
Part 2: The individual organism is a self-maintaining and self-replicating system that transforms inputs (light, air, water, food) into outputs (energy, new cells, and other things). Complex organisms are systems which themselves contain subsystems such as the respiratory, circulatory, and digestive systems, all of which interact in a manner that preserves the existence of the main organism and also (at least from time to time) generates new instances of the same species (i.e., children). Each subsystem also transforms inputs into outputs, and the outputs of one subsystem are generally inputs into another.
Example: One of the outputs of the respiratory system is oxygen transferred as an input to the circulatory system which then outputs the same oxygen to individual cells which use it in combustion whose output is energy. One of the outputs of the digestive system is glucose transferred in the same way as an input into the circulatory system for use as a fuel in the previously-mentioned combustion at the cellular level. Then the energy produced by the combustion at the cellular level is used to run all the other activities of the body, and so on.
Characterizing an organism as a system composed of subsystems is one way to describe it. But an organism can also be described in terms of its characteristics or features. These features are consequences of the many subsystems that comprise the organism, but often in a holistic way -- that is, a feature like height or strength or speed is generally not the output of a single subsystem but the result of the cooperation of many subsystems.
That said, however, it is a basic principle of Darwinian theory that all or most of the features or characteristics of an organism are somehow derived from an evolutionary advantage or survival benefit which they conferred at one time or still confer. (The qualifier "or most" is there because sometimes a feature is a random mutation. The qualifier "at one time" is there because in some cases there appear to be features which have outlived their usefulness, like the vestigial leg bones in whales or like the coccyx in humans.) Therefore, some features of an organism can be described as contributing to the organism's survival in an evolutionary sense. An example is the swiftness of the gazelle, which allows it to outrun predators. There might of course be -- and in some cases there certainly are -- other features of the same organism which do not contribute to its survival.
In this context, the purpose2 of any feature or subsystem of any organism is defined to be the function that it plays in maintaining and reproducing the organism as a whole (for subsystems), or in ensuring its survival and reproduction (for features). Since the whole point of Darwinian theory is that the development of species can be characterized in terms of evolutionary advantage, it should be clear that Darwinian biology guarantees the existence of purpose2 for any species.
Part 3: In some organisms, some features can be used in a variety of different ways. For example I can use my fingers to bring food up to my mouth, but I can also use them to type on the keyboard of my computer. Now it should be clear that while I do derive some personal advantage from being able to type on a keyboard, there is no possible way that the ability to type on a keyboard is a usage which provided enough survival benefit to lead to the development of fingers in the human species. Personal computers came into existence only in my lifetime, and even typewriters did not exist when my grandparents' grandparents were born. But people had fingers long before that. So when an organism uses one of its features in a way that clearly played no part in that feature's evolutionary development, I call that an "adventitious" usage.
The opposite of an adventitious usage is a usage that was driven by evolution. I call this kind of usage an "evolutionary" usage. (For all I know there might be middle cases which are neither evolutionary nor adventitious under the current definitions, but that is not a problem for the argument.)
Example: When a gazelle uses its high speed to escape from a leopard, that is an "evolutionary" usage of the feature of high speed. On the other hand if two gazelles were simply playing "Tag" with no predators in sight, that would be an adventitious use of the feature of high speed. (I have no idea whether gazelles play "Tag" in real life.)
I now define the word purpose3 as a shorthand, such that the sentence "Behavior B is an evolutionary usage of feature F in this species" is synonymous with the sentence, "This species has F for the purpose3 of B." For the sake of variety I will also introduce the phrase "in order to" which I will define so that the same sentence can also be phrased as, "This species has F in order to B."
Example: Gazelles are fast in order to escape from predators. Cats have curved claws and strong pouncing muscles in order to catch mice.
Again, it should be clear that any organism which can be described as having any kind of behavior must (according to Darwinian biology) have some behaviors which meet the description of purpose3. In other words, there might be room for disagreement over whether you can discuss the "behavior" of algae. But in all contexts where the term is meaningful, Darwinian biology logically entails the existence of purpose3.
Part 4: To recapitulate where we are so far, any species S can be described as having a variety of features or characteristics. And in general we can suppose that any feature F can be used in a variety of ways, some evolutionary and some adventitious. When I talk about any individual of S using F in a certain way, I am talking about different kinds of behavior. A usage is a kind of behavior. So if I talk about a set of behaviors, I can also talk about the subset of those behaviors which constitute a usage of some feature of the species.
Therefore, for some species S, consider the set L of all behaviors available to S which are also evolutionary usages of some feature F of S. I now define the phrase "natural way of life" so that L is the natural way of life for S. Basic Darwinian theory ensures that L is never an empty set, because every kind of organism has some behavior which has been determined by evolution. Therefore Darwinian theory guarantees that every species has a natural way of life.
QED.
The next step is to start inquiring about what exactly constitutes the natural way of life for Man, but this has taken me long enough that I want to stop here and rest for a bit. Subsequent installments will recapitulate the argument of this post and this one ... and perhaps will move forward from there a little as well.
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