There is no therefore symbol in the default LaTeX packages. There are (at least) two ways of getting the symbol:

The package amssymb contains the therefore symbol definition. To use, do something like:

usepackage{amssymb}
...
$therefore$

Alternatively, you can create your own therefore definition:

deftherefore{
leavevmode
lower0.1exhbox{$bullet$}
kern-0.2emraise0.7exhbox{$bullet$}
kern-0.2emlower0.2exhbox{$bullet$}
thinspace}

The amssymb therefore has tiny dots, whereas the definition above has large dots

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