An abstract bit is an element of the set [boolean $~$\mathbb B$~$], which has two elements. An abstract bit is to $~$\mathbb B$~$ as a number is to [natural_number $~$\mathbb N$~$]. The set $~$\mathbb N$~$ contains a bunch of "numbers", and these numbers sometimes have many different labels, such as "three" and "3". Similarly, the set $~$\mathbb B$~$ contains a pair of "bits", which sometimes have many different labels. The abstract bits are sometimes labeled 0 and 1; true and false; yes and no; or + and -. The labels don't matter; the point is that the set $~$\mathbb B$~$ contains two elements, and we call those elements (abstract) bits.