Why is redefining a command erring? [closed]

Question

I am trying to redefine \emph to bold its argument:

\renewcommand\emph[1]{{\textbf{\oldemph{#1}}}}

However, after typing this command before begin{document}, I get an error every time I use \emph{Some text.}: Undefined control sequence.

What is a control sequence? How do I fix my redefinition?

Answer 1

The following works for me

\documentclass{article}

\let\oldemph\emph
\renewcommand\emph[1]{{\bfseries{\oldemph{#1}}}}

\begin{document}

\emph{hello world}
\end{document}