How can I explain the uses of the definite article in these sentences?

Question

How can I explain the uses of the definite article in these sentences?

"The school teaches blind people to read"
"Louis Braille created a writing system to allow the blind to read"

Answer 1

The first sentence probably is from a larger news column or book, where some specific school is being discussed.

As for the second sentence, forget that silly notion which says the definite article is only used for something that is specified or was mentioned beforehand. That's only one of several uses for the definite article; this dictionary lists almost a dozen, including this one:

Used before an adjective extending it to signify a class and giving it the function of a noun: the rich; the dead; the homeless.

That's exactly what is going on when your sentence talks about "the blind."

I discuss this in more depth in an earlier answer to a similar question.