Latex: Numerar párrafos

Tomado de:
http://stackoverflow.com/questions/543208/how-can-one-number-paragraphs-in-latex

\newcounter{parnum}
\newcommand{\N}{%
   \noindent\refstepcounter{parnum}%
    \makebox[\parindent][l]{\textbf{\arabic{parnum}.}}}
% Use a generous paragraph indent so numbers can be fit inside the
% indentation space.
\setlength{\parindent}{2em}

se pone antes del párrafo \N