# Constant Quantities And Constant Domains

When explaining category theory to my friends (which happens more than you might think), I often find myself summing the theory up as “the abstract study of variable quantities”. We analyse the notion of a variable quantity into two components: what the quantity is, and with what the quantities varies. To say that a quantity $q$ is an number that varies with the time (say, if $q$ is the position of a beetle as it walks along the edge of a ruler, or the angle in degrees that the sun makes with the horizon), we write $q : T \to N$ where $T$ is our notion of time and $N$ our notion of number.