Applying Inductive and Deductive Logic to the Theory of Evolution

In a previous post, I explained the difference between inductive logic and deductive logic and how we use both types of reasoning in science (I recommend that you read that post before continuing). In this post, I am going to expand on that by demonstrating how we used both types of logic to arrive at the theory of evolution.

When Darwin constructed his theory, he began with deductive logic.

  1. Each of the Galapagos islands has its own species of tortoise
  2. The tortoises on each island are specialized for the particular habitat of that island
  3. These islands have not been present for the entire history of the earth
  4. There is a mainland tortoise that is very similar to these species, but none of these species are actually found on the mainland
  5. It is highly unlikely that at one point all of these species were on the mainland, and all of them got washed out to these islands, but only one or two species survived on each island, and all of them went extinct on the mainland
  6. Therefore, these species must have evolved from a common ancestor that drifted from the mainland

This conclusion was, of course, later supported by modern genetics, and even creationists accept Darwin’s claims on this one. This is, in fact, what creationists have termed “microevolution.” It is what Darwin did next that has creationists so upset.

  1. For many species from all over the world, we can see patterns like the Galapagos tortoises
  2. From breeding experiments we know that almost endless variations of an organism are possible
  3. There is no reason to think that there is a limit on this ability to evolve (i.e., organism should be able to accumulate in infinite number of small changes which when added together would equal many very large changes)
  4. We can see evidence of progressive changes in the fossil record
  5. Therefore, all life on planet earth evolved from a common ancestor

Notice, this is not faith, this is inductive logic. The real support of evolution of course came later. Darwin’s inductive conclusion made numerous predictions that all came true. For example, it predicted modern genetics and the biogeographical patterns that we are familiar with today. This is the strength of the theory of evolution, it makes predictions, and those predictions come true.

To conclude this post, I want to bring the concept of inductive logic to bare on the faulty argument that scientists accept that the earth is old on faith, or “assume” that decay rates are constant.

  1. Every time that we have ever measured radiometric decay rates they have been constant
  2. There is no logical reason to think that they are not constant and there are strong mathematical/logical reasons to think that they are constant
  3. Numerous experiments only work because radiometric decay is constant
  4. Therefore, radiometric decay rates are constant

In fact, the conclusion is simply the universal law of radioactive decay which states that decay rates are constant and uses mathematical formals to predict the amount of decay in a given item. Yes, you read that right, the idea that decay rates are steady is a scientific law (note: when I say “constant” or “steady” I mean that the half-life of an element does not vary with time, the actual decay of a given item is exponential).

My point here is simple, scientists are not “believing” or “assuming” that evolution is true, decay rates are constant, etc. any more than we are “believing” or “assuming” the theory of gravity or the laws of thermodynamics. Rather, we are accepting the conclusion of inductive logic, and for a creationist to say that we don’t have to accept radiometric dating because it assumes a constant decay rate is no different from saying that we don’t have to accept gravity because we are assuming that its universal. Evolution and the age of the earth are science based, not faith based.

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