It's alief vs. belief. It's one thing to see that, in theory, almost all spiders are harmless. It's another to remain calm in the presence of a spider if you've had a history of being terrified of them.

Desensitization is a process of teaching a person how to calm themselves, and then exposing them to things which are just a little like spiders (a picture of a cartoon spider, perhaps, or the word spider). When they can calm themselves around that, they're exposed to something a little more like a spider, and learn to be calm around that.

The alief system can learn, but it's not necessarily a verbal process.

Even when it is verbal, as when someone learns to identify various sorts of irrational thoughts, it's much slower than understanding an argument.