This is the easiest way I have found to typeset premise and conclusion style arguments in LaTeX. First, add this to your preamble:

\usepackage{amssymb}

Then use something like this to typeset the argument:

\begin{itemize}
    \item[1.] Premise One.
    \item[2.] Premise Two.
    \item[3.] Premise Three.
    \item[$\therefore$] Therefore, Your Conclusion.
\end{itemize}