darwin & the hypothetico-deductive method

Charles Darwin published his Origin of Species knowing that it was unreasonable to demand demonstrative proof of his theory of natural selection because it acted over such long periods of time. Scientists who opposed Darwin expected from him absolute proof and absolute certainty of his theory. Perhaps they wanted a direct observation of species evolving over an observable time frame, a fossil record with no gaps that showed every stage of species transition, or some kind of experimental demonstration of natural selection occurring. Historian Peter J. Bowler holds that no philosopher of science would accept this critique because we now view science as fallible. This fallibilist view of science implies a kind of tentativeness inherent in science, and emphasizes that scientific knowledge can be revised as new evidence is brought to light. This is in contrast with infallibilism, which suggests that science can provide us with theories that guarantee a certainty about the world. I argue that Charles Darwin used what we now call the hypothetico-deductive method to test his theory. Further, any challenge which aims to undermine the scientific nature of Darwin’s theory based on his usage of this method or utilizes the infallibilist position that this method is unscientific because it cannot guarantee truth is invalid.

First we must discuss what the hypothetico-deductive method is and what it entails. The method is a way to systematically approach scientific inquiry, which involves forming a hypothesis, deducing predictions or retrodictions from the hypothesis (usually in the form of if…then…), conducting experiments or observations to test these predictions, and applying inductive reasoning to evaluate whether the outcomes of the experiments or observations align with the hypothesis. This method offers a potential solution to the problem of induction as popularized by philosopher David Hume, who argued that induction does not allow us to come to general claims based on specific observations. A famous example used by Karl Popper is the following: say we have only seen white swans in our lifetime. Employing induction, which aims to create a general theory given particular observations, we may believe that all swans are thus white. However, the existence of a black swan immediately falsifies our hypothesis. The significance is that no amount of evidence can ever guarantee a theory to be true, because all it takes to falsify it is a single counterexample. Scientists who opposed Darwin who emphasized a need for direct evidence that they thought guaranteed scientific truth may have been implicitly leaning towards an inductive process, such as an accumulation of evidence about species evolving. Popper places an extreme emphasis on falsifiability as a solution to the demarcation problem (distinguishing real science from non-science) and the induction problem: a scientific theory must be able to be proven wrong. The hypothetico-deductive method emphasizes deduction by encouraging scientists to make predictions and retrodictions, and emphasizes falsifiability by encouraging them to conduct experiments and observations to test whether their deductions were correct. However, it is important to note that this method does not guarantee scientific truth either: testing that a hypothesis cannot be falsified through various means does lend credibility to the theory, more so than just evidence for the theory through an inductive process would, but it does not guarantee truth.

Now we can show specifically that Darwin was using this method by going through each step of the hypothetico-deductive method sequentially and showing that Darwin took each step:

1. Forming a hypothesis: Darwin hypothesized that species went through transformations through the process of natural selection.

2. Deducing predictions or retroductions: Darwin predicted that if populations of species became isolated in different environments, natural selection would affect them differently and thus the species would look different after many generations.

3. Conducting experiments or observations: Experimentation is very difficult with respect to evolution because the time for species to evolve with noticeable changes takes many generations and many years. So, Darwin mainly conducted thorough observations. For example, he traveled to the Galapagos islands to observe different populations of finches which had been isolated from each other on different islands. He noted that each island had different environments and saw that finches on each island had variations, particularly in the shape of their beaks.

4. Applying inductive reasoning: He noted four subgroups of finches, each suited to their environment. Ground finches had short and thick beaks which allowed them to crack the hard nuts found in their corresponding island, cactus finches had long, pointed beaks which helped them feed from cactus flowers, tree finches had medium sized beaks which allowed them to eat insects and fruits, and warbler finches had thin beaks which helped them feed on insects. He compared these variations to the environments the finches lived in, and induced that they aligned with his theory.

Now that we have shown that Darwin was, in fact, using the hypothetico-deductive method, we must show that a challenge to Darwin’s theory that aims to undermine the scientific nature of this method is invalid. In order to do this, we must show that the method itself is scientific. I hold that a method is scientific if it aligns with the scientific method as outlined by philosopher Charles S. Peirce, which I find to be an exemplary explanation of how we ought to pursue scientific inquiry. Peirce defines the process as having three key steps: abduction (the process by which scientists create a hypothesis that they will later test), deduction (inferring what should be the case if the hypotheses are the case), and induction (testing the hypothesis). We can now show that the hypothetico-deductive method aligns with the scientific method by going through these three steps:

1. Abduction: This is trivial, as the first step of the hypothetico-deductive process is coming up with a hypothesis that is the best explanation of phenomena involved.

2. Deduction: This is also quite trivial. The second step of the hypothetico-deductive method is deducing predictions or retrodictions in the form of if…then… statements.

3. Induction: The final two steps of the hypothetico-deductive method are conducting experiments or observations and applying inductive reasoning to evaluate the hypothesis. These serve as the test of validity of the hypothesis meant by Peirce’s final stage of the scientific method.

So, we can see how the hypothetico-deductive method completely aligns with the scientific method, and thus is scientific. We must also show that a challenge to Darwin’s theory based on his use of the hypothetico-deductive method to test it because that method of testing cannot guarantee the truth of the hypothesis is also invalid. To prove this, I turn to my earlier discussion about the induction problem and Popper’s criterion of falsifiability. There is no scientific process that can yield absolute truth: the inductive process cannot because we always run the risk of finding a counterexample, and the hypothetico-deductive process cannot for the same reason. The difference though is that the latter process has a dedicated stage (experimentation and observation) to find the evidence that might falsify the theory. The more chances we give a theory to be falsified, the more sure we can be of its truth when it passes these tests. This critique is not reasonable because we can never guarantee truth by any method — at least the hypothetico-deductive method lends us more credibility by allowing the theory to withstand so many chances at falsifiability.

We can imagine a counter argument which takes issue with the idea that Darwin is using the hypothetico-deductive method even though he is in a situation where he cannot necessarily check his predictions. For example, Darwin deduced that, if his hypothesis were correct, we would find intermediate species in the fossil record which showed transitional stages. However, he never actually found something like the wooly mammoth fossil, which looks similar to modern Asian elephants, and thus could have been evidence to support his hypothesis. This is true, and a valid counter argument — it would have been stronger for Darwin’s argument if he had found this evidence. However, Darwin did as much as he could at the time. I hold that scientific knowledge is provisional; that it is the best we can do at the current moment. Darwin was limited by his lack of knowledge about genetics, incomplete fossil record, and lack of technology, and had to make do with what observation he could conduct at the time — it was still scientific.