Singleton (set theory)/Definition

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Definition

Let $X$ be a set. We call $X$ a singleton if:

• $\exists t[t\in X\rightarrow\forall s(s\in X\rightarrow s\eq t)]$
  • In words: $X$ is a singleton if: there exists a thing such that ( if the thing is in X then forall stuff ( if that stuff is in $X$ then the stuff is the thing ) )