3.1 Someone

Let's think about the following sentence:

``Someone saved Tom from drowning.''

This means that an entity (a person, a dog, a dolphin, an angle,...) saved Tom from drowning. Better yet, we can say that there existed an entity x, such that x saved Tom.

``There existed an entity x, it is true that x saved Tom from drowning.''

The mathematical symbol to use here is $\exists$. We can now rewrite our sentence as follows, assuming $\mathrm{P}(x)$ means ``x saved Tom'' and $\mathrm{E}$ is a set of all entities:

$\exists x \in \mathrm{E}: \mathrm{P}(x)$

The colon means ``it is true that''. The symbol $\in$ means the left hand side is in the right hand side.