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