When you say, "The abnormal mind is quick to detect." It appears to me that you are referring to a certain specific person who has that brain because the word "The" is in a demonstrative form. meaning that the abnormal mind is something that you already mentioned or completely specified later in that same sentence.
When you say, "An abnormal mind is quick to detect." The word "An" refers to an indefinite object, meaning that the word "an" is used when the noun may refer to different possibilities, and to something not specified.
Let's review:
Indefinite nouns - can have many meanings, meaning it does not refer to any specific objects. Nouns that are preceded with "a" or "an" article is indefinite.
A dog
An egg
An Alligator
Definite nouns - are nouns that are specific for the speaker or to the listener, it precedes with the word "The"
“I’m inclined to reserve all judgments, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores.
The abnormal mind is quick to detect and attach itself to this quality when it appears in a normal person.”
- The narrator used the word "The" because he is pointing at his own brain and describes what his brain does when it appears in a normal person. Or if there's an abnormal mind already mentioned before that part of the sentence, he's probably referring to someone's brain.
Hope this helps.
