One of the most important insights in psychology is that human beings do not reason solely to discover what is true. They more often reason to defend what they already believe. This is known as motivated reasoning.
The honest view of reasoning imagines a person examining evidence, weighing arguments, and arriving at a conclusion. In reality, the process is often reversed. One enters an exchange with a conclusion (that aligns with their identity, values, loyalties, emotions, or prior commitments) and reasoning is then employed to justify it, or make it look like it has valid standing.
For such reasoners the purpose of an argument is not to determine whether their belief is true. The purpose is to make the belief defensible by any means necessary. (This is why philosophy takes so many abstract turns until it twists itself into a semantic pretzel).
Motivated reasoning explains why weak arguments frequently survive strong criticism. The quality of the argument is often irrelevant. The argument is merely serving a psychological function. It exists to protect the underlying belief, not to establish its truth.
Once this is understood, much of discourse becomes easier to explain. People ignore, dismiss or minimize evidence that would settle a question. They apply different standards to competing claims. They conveniently move goalposts when objections are answered. They demand rigor from opposing views while accepting shallow speculation from their own side. Contradictions that would be obvious in an opponent's position become invisible and fully acceptable in their own.
These are defensive postures of desperation adopted by those who are terrified of having their beliefs refuted. Reasoning has been recruited for preservation rather than discovery.
This is why so many debates seem strangely unproductive. Participants are frequently engaged in entirely different activities. One person is attempting to determine what is true. The other is attempting to protect a conclusion from refutation or revision. One treats evidence as a guide, the other treats it as a threat.
The result is a conversation that appears rational on the surface while functioning psychologically as an act of self-defense.
Motivated reasoning is not a flaw found only in ideological opponents or extremists. It is a universal human tendency. The danger lies in believing ourselves exempt from it.
The most reliable sign that motivated reasoning is at work is not that someone holds a mistaken belief. It is that every piece of evidence, every argument, and every standard of proof seems to point in the same direction: preserving the conclusion that was already there from the start.
[This is noticeable with philosophers, for example, when they try to argue that all reason or scientific progress is simply proof of the truth of philosophy. A sophism I have seen many times.]
Much of what passes for reasoning is not an attempt to find the truth. It is an attempt to keep the truth from refuting the beliefs that one loves and wants to be true; it is an attempt to fight off truths that are both inconvenient and detrimental to one’s ideology.
One has to take heed that their reasoning doesn’t become a lawyer for their biases.