System

A system is defined to be a collection of things that are related to each other. Therefore, all systems can be defined as a tuple of set of things, and a set of relations.

A system $S$ can be defined as a tuple: $\{T,R\}$, 

where

$T$ is a collection of things, $T=$Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \{ t | t \in T}}
 and 
$R$ is a set of relations that relates two things $t\in T$.