CT Scan of the Human Brain
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So while their excuse may make it sound like their behavior or ideas were arrived at on the basis of reason and evidence, the truth is that their behavior or ideas were arrived at on an unconscious level, so maybe reason and evidence didn't play a role. Indeed, there are cases where reason and evidence definitely didn't play a role, but the person still tries to make it sound reasonable.
“A gazillion experiments show that I can flash something at you so fast you don’t see it, yet the information does bias you towards one decision as opposed to another,” says cognitive psychologist Michael Gazzaniga of the University of California at Santa Barbara.
Moreover, he says, the literature is full of experiments showing that conscious explanations for our behaviour are often just rationalizations invented after the fact. He cites the example of a patient whose corpus callosum had been severed as a treatment for epilepsy, making it impossible for one side of the brain to communicate with the other.
Gazzaniga and his colleagues presented the word ‘walk’ to the patient’s left visual field, which corresponds to the right side of the brain. When the patient stood up and began walking, they asked him why; the right side of the patient’s brain had been shown to lack the ability to process language. His left brain, which never received the walk command, but which handles language processing, quickly invented a logical explanation: “I wanted to go get a Coke.”
Source: Nature, Vol 457, 29 January 2009
Now, if we arrive at a decision at an unconscious level without the benefit of reason, logic, and evidence, but then offer a completely sensible, reasonable, logical justification for it, is the position rational or not? Maybe we didn't originally reach the decision on the basis of reason, but that wouldn't change the fact that it can be justified rationally. It should be possible for our decision to be irrational, but the position itself to be rational, right?
This makes the ability to support a claim or decision so important. We really can't know for sure if our reasoning process is truly bringing us to a rational decision or if, instead, it's just our brain's way of rationalizing a decision it already reached on an unconscious level without our realizing it. If we can support the final position through reason, logic, and evidence, though, then our original, unconscious decision-making process need not be treated as having tainted the final position.