Therefore sign

If you want to do this often, you can create a keybindings file in your Library to map it to a key combination.

In ~/Library create a directory named KeyBindings. Create a file named DefaultKeyBinding.dict inside the directory. You can add key bindings in this format:

{
    "x" = (insertText:, "\U23CF");
    "y" = (insertText:, "hi"); /* warning: this will change 'y' to 'hi'! */
}

The LHS is the key combination you'll hit to enter the character. You can use the following characters to indicate command keys:

@ - Command

~ - Option

^ - Control

You'll need to look up the unicode for your character (in this case, ∴ is \U2234). So to type this character whenever you typed Control-M, you'd use

"^m" = (insertText:, "\U2234");

You can find more information here: http://www.hcs.harvard.edu/~jrus/site/cocoa-text.html

For Apple apps, make a text replacement for it:

https://support.apple.com/kb/PH18439

Other apps like MS Word have their own system for this in their menus.