Mathematics
The Duality of Vector Spaces and Linear Functionals
Quick fact
The dual space of a finite-dimensional vector space has the same dimension as the original space, but the two are not naturally identical: they are 'dual' to each other.
Why this is interesting
Imagine you have a vector, and you want to 'measure' it in a linear way—how can you do that, and what happens if you consider all possible measurements at once?