This question already has an answer here:
If we can say that any positive rational number is countable or listable by showing that every positive rational number is the quotient of
p/q of two positive integers, and then listing those in an array like this:
1/1 2/1 3/1 4/1 ... 1/2 2/2 3/2 4/2 ... 1/3 2/3 3/3 4/3 ... ....
We know that each row can go on for infinity, so instead we can draw a diagonal lines through the array to ensure that every fraction is counted, right?
But if we take all real numbers and try and list or count them, allegedly we can't, but why?
For example, if we wanted to list or make countable all numbers between 0 and 1, why can't we say that all possible permutations of decimal number combinations could be listed or counted the same way that all fractions are countable or listable for positive rational numbers?
If we're unable to list or count all possible combinations of decimal numbers between 0 and 1, why are we not saying that all the infinite combinations of fractions used to make up all positive rational numbers isn't countable as well?
I realize that a very erudite and technical explanation of the Cantor Diagonalization Argument was provided in this post, however I'm not specifically asking for an explanation of this argument and moreover the very thorough explanation provided by Arturo Magidin for the OP in the post is slightly beyond my understanding as I'm very new to set theory and discrete mathematics.