A discussion of unanchored backward chaining. Originally posted on Quora, but I have since been banned.
Suppose that I want to show that my neighbor does not have an elephant in his back yard. I can do this by first noting that elephants are very large, make a lot of noise, and leave footprints, especially if the ground has been wet. Is this an absolute fact? No. Maybe there is an elephant somewhere that does not have any of these characteristics. But we have made quite a few observations and infer that elephants have these features. But to make sure, I ask my neighbor if he agrees with these statements. If he does, I can proceed. This makes the list of assertions a well established foundation.
Now that I have this foundation, I can make a series of observations in the neighbor’s back yard. Suppose I haven’t seen any big animals towering above the measly fence which separates our two yards, that I haven’t heard any noise coming from my neighbor’s back yard, and that I see no footprints when I walk through the yard, even though it rained a few days prior.
I can add these observations as a second layer on top of my foundation, and from that, add the top layer: the conclusion that my neighbor does not have an elephant in his back yard.
It is pretty easy for a human to reach a conclusion from a given set of assertions, by working forward. In computer science, it is sometimes easier to work backwards, starting with the conclusion, and through various substitutions of the initial assertions, determine whether or not the conclusion is true or not, based on those initial assertions. This is a process called backward chaining and it is a valid process.
Unanchored Backward Chaining
Now here is where things get interesting. In the initial elephant example, I more or less worked in a forward manner. Sure, I was thinking about what foundation would be useful in determining whether or not the conclusion was true, but I made sure my foundation was solid and then made sure that I could build from that foundation to my conclusion. Before starting my argument, I made certain that the person, with whom I am arguing, agreed with the initial assertions, and then used those assertions as an anchor for the remainder of my argument.
But many arguments relating to religious claims work in a very different way. First, a conclusion is asserted to be true and the goal is to “prove” the statement true. Now, instead of working on an established foundation, a claim is made which leads to the conclusion that the original claim is true. But there is no substantiation for that second claim. Once pressed, the person seeks to find another claim which can then substantiate the second claim, and so on. Because there is no solid foundation from the start, I call this process unanchored backward chaining.
Creationism and Anti-theism
Two instances in which unanchored backward chaining is common are in arguments relating to creationism and arguments relating to claims that there are no gods. Both start with their respective conclusions and try to progress backwards.
As an example, creationists sometimes rely on a concept known as “irreducible complexity.” The idea behind the irreducible complexity argument is that some features of life are simply too complex to have evolved randomly. Certainly if this is the case then creationism or intelligent design are more or less the only explanations for life existing in its current form.
But for irreversible complexity to hold, there would have to be no paths available through which evolution can produce a given result. As such, the claim that life is irreducibly complex is unsubstantiated. But for the average creationist, they are comfortable with the level of argument and therefore feel satisfied that they have indeed substantiated the claim. Meanwhile to someone who does not support creationism, they would continue to ask for the next link in the chain.
We can see the same process in attempts to discredit the existence of a god. One example is the following assertion: “The fact that no God has contacted us in an unambiguous manner is contrary to the nature of God as depicted in most religions and therefore evidence that such a God doesn’t exist” (Glen Tarr, Quora answer, accessed October 15, 2015).
Is this substantiated? It doesn’t seem to be. Why is it contrary to the nature of God, as depicted in most religions? Indeed, the Christian god seems to want people to believe without evidence rather than when presented with clear evidence. This is apparent from the story of doubting Thomas. True, Christianity is just one of many religions in the world, but it is a very prominent religion.
Another chain can be added to this one: god wants certain things from us and this is why a lack of unambiguous communication is evidence against a god (Glen Tarr, comment discussion, October 14, 2015). But why would this be the case? Because it is easier to get humans to do what such a god wants them to do? Certainly, but even using that claim is just another link in the chain. Why would a god act in a way which is easiest?
Validity of Use
This does not mean that unanchored backward chaining “wrong.” It can be used when investigating various claims. Indeed, you may need to use this process, in order to find an anchor link and determine whether or not the claim seems reasonable. But an unanchored link should not be used within an argument because it is far too easy to undermine an argument which relies on it. Once an anchor is established, the only way for a opponent to invalidate the conclusion is to break a link in the chain: show that one of the links is wrong. With an unanchored chain, all an opponent has to do is reject the newest link. This, combined with the initial desire to prove the conclusion right, results in a possibly endless search for evidence to support the conclusion.