Philosophy of science courses should be taken more seriously at colleges and universities, but they can run into difficulty when they address the complicated, messy subject of biology. My old professor, colleague and friend, Jack Maze, points out that biology, unlike physics and chemistry, is highly variable, rich in data but poor in theory. Sometimes a philosophy of science course is brilliant, sometimes (generally) not. I was fortunate to have had the opportunity to absorb some of Cy Finnegan’s History and Philosophy of Biology at the University of British Columbia when I was a graduate student. This enthralling class explored the ideas of several philosophers and historians and focused with some intensity on the structure of causal scientific explanations. To anyone wishing to understand the difference between theory and not-theory, especially in biology, the consideration of the structure of scientific explanation is important.
Science bases explanations on things called “laws,” or the regular (sometimes partly predictable) behaviors and interactions of the forces and substances in the universe, providing the constraints that shape natural phenomena. These laws offer partial prediction in that they describe a pattern of events and relate them to a single causal source, though the specifics of those events may not be fully knowable until they occur. They are called laws because they seem never to be violated, but our understanding of them has often been (and continues to be) crude, or even incorrect. Science can’t explain beyond the point of the laws of nature because these form the “ultimate” set of actions and behaviors in our causal explanations. Religion, of course, can “explain” whatever it likes beyond the laws of nature and without benefit of evidence. It is entirely possible that the laws of nature were created and set into motion by a deity (I think this is part of the definition of Deism), but it is not possible to scientifically test (or even approach) such an idea. In order to do science, the boundary between it and religion must be clearly marked, but science and religion need not be in conflict.
The philosopher Carl Hempel (one reference among others is Hempel’s Philosophy of Natural Science, 1966, Prentice-Hall, Inc.) developed a logical framework that can be used to examine scientific theories and hypotheses by making explicit how, given specified initial conditions, and given a natural law, events are caused. This logical framework, called Hempel’s “Covering Law Model” recognizes the initial conditions and the law as the explanans, or the things that, when brought together, cause the explanandum (the event) to occur. The links or bridges between explanans and explanandum form a logical, causal relationship. For instance, if I drop a rock from a short distance above the Earth’s surface (the initial conditions), gravity (the law) will cause the rock to spontaneously move toward the Earth’s surface (the event). My friend Ruth Deery suggests other examples at the finer scales of chemistry, physics and biology: the egg shells of birds of prey become thin (the explanandum) when DDT accumulates in their tissues (initial condition) and interferes with normal egg shell development (regularities of chemistry and bird physiology). As greenhouse gases increase in the atmosphere (initial condition), they trap heat escaping from the Earth (laws of chemistry and physics), causing events such as a rise in sea levels and the flooding of coastal cities.
The Covering Law Model provides a structure for looking at and testing hypotheses and theories with relative clarity, demanding identification of the conditions, laws and event(s). In other words, by distinguishing between that which explains (the laws and conditions of the explanans) and the explanandum, or the event to be explained, clarity is greatly improved.
Another important component of the Covering Law Model is the “bridge principles.” These are the logical connections between the conditions and laws of the explanans, and also the links between the explanans and explanandum. These bridges are necessary parts of the structure of theories because they insure that the causal explanation is actually relevant to the event being explained. For example, I will argue that Darwinian and neo-Darwinian (incorporating genetics) theory lacks coherence when the parts are viewed within a Covering Law structure. Jack Maze points out, “Interestingly enough, it is the lack of explanatory cohesion in Darwinian and neo-Darwinian theory that some creationists exploit in their attacks on evolution.” I would add that, being creationists, they are not required to reciprocate and can offer whatever “explanation” pleases them. Theories and the events they explain must form at least a partly coherent whole. Even if there are vague areas, the connections must have some clarity, be logically possible and empirically testable (though testing is not always direct). It is not a question of neo-Darwinian theory (science poorly done) versus “creationism” or “intelligent design” (not science at all). There is more than one theory or causal explanation for evolution, and some make far better candidates than others in terms of logical connections and relevant explanatory power. Scientific explanations are more than mere assertions. Their constraints are demanding and rather rigid, but there is room for explanations to change and evolve within these constraints. “Creationism” or “intelligent design” doesn’t play by the rules of science, so it doesn’t get into the ballpark. Rationality and faith are not interchangeable, and bacterial flagella do not provide proof of an interfering deity.
It’s interesting to think about how our understanding of the laws of nature changes. The example of the dropped rock works for just about any version of the law of gravity – Newton’s, Einstein’s or a more recent version. We would have to try different experiments to distinguish between the different descriptions of gravity – as an attractive force between two masses or as masses distorting the fabric of space, or other possibilities. The rock still falls to the ground, but our understanding of why this event occurs has changed more than once and will no doubt change again. There is a certain level of prediction here as well – we can predict that, given the initial conditions, the event will occur. However, in many cases we cannot predict the full details of the event. This unpredictability may not be due to the failure of science, but may instead be another property of nature, a property called emergence. A wave or convection current in water has properties beyond those of the water molecules. A convection current can be neither predicted nor described from the molecular properties of water – the current expresses a different level of organization, and it appears spontaneously when energy (heat in the case of convection currents) flows through the watery system.
Another philosopher of science, Israel Scheffler (Explanation, prediction and abstraction. in Danto and Morganbesser, eds., Philosophy of Science. 1960. Meridian Books), made an important point about predictability when commenting on the Covering Law Model. He argued that although the model could be used to make predictions in many cases, explanation was its most important function. It is not necessary to be able to predict in science; it is necessary to be able to explain, logically and as explicitly as possible. The Covering Law Model provides for this, offering a structured and clear way to think about theories and hypotheses, to find their strengths and flaws without requiring that they be fully predictive.
To say that prediction is not an important aspect of science runs counter to some versions of the “scientific method,” as it is often taught: make predictions as part of the hypothesis, collect data (usually after manipulation of a single variable), test hypothesis, and reach conclusion as to success of predictions. But what if some aspects of nature are simply not predictable? Is it reasonable to think that we could predict all the details of a new species that has not yet evolved? No. So, does this mean that phenomena such as evolutionary events are scientifically unapproachable? Not at all – Scheffler had an excellent point. The idea that all natural events should be fully predictable is related to the idea of the universe around us functioning as a steady machine such as a watch. Various references to a “watchmaker” (God) and sometimes to a “blind watchmaker” (the natural selection of Darwinian or neo-Darwinian theory) have been common throughout the brief history of western science. Perhaps the analogy of the universe as a watch is misleading. A better analogy might be to think of the universe in terms of emergent properties; as a molecule of salt, or a convection current, or perhaps as a developing, living thing.
Jack Maze and Cy Finnegan offer a final clarification: “To those who may recoil in horror from the statement that prediction is not an important aspect of science, I would offer the following. Prediction can be used in two ways. One, as emphasized above, is a general statement that incorporates natural law (in describing) what may happen in the future. The other is as part of hypothesis testing. Hypothesis testing has the form of “if-then” statements: if a certain condition exists, then a certain observation is to be expected, is predicted. This is a different use of prediction and is often the one inferred when doing science.”