How to have indented text in ntheorem environment without a newline at the beginning?

Question

I would like to have an indented paragraph inside a ntheorem environment where the first line is not indented. Using the quote-environment inside ntheorem lets the paragraph start with an ugly indentation after the theorem name, like this:

\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath,thmmarks]{ntheorem}
\usepackage{blindtext}
\newtheorem{proposition}{Proposition}

\begin{document}
\blindtext
\begin{proposition}
\begin{quote}
\blindtext
\end{quote}
\end{proposition}
\blindtext
\end{document}

Which gives leads to:

Is there a way to remove the space between "Proposition 1" and the beginning of the paragraph? \noindent just lets the paraphraph start in a new line.

Answer 1

quote is a built on a list environment, and what you see is the space of \leftmargin followed by \itemsep. To counteract this, back up by these amounts.:

\documentclass{article}

\usepackage{amsmath}
\usepackage[amsmath,thmmarks]{ntheorem}
\usepackage{blindtext}

\newtheorem{proposition}{Proposition}

\begin{document}

\blindtext

\begin{proposition}
  \begin{quote} \hspace*{\dimexpr-\leftmargin-\itemsep}
    \blindtext
  \end{quote}
\end{proposition}

\blindtext

\end{document}

A similar approach should work for other list environments.

Answer 2

If you are using the quote environment inside the proposition solely to get the text indented, you could set \theoremindent instead. Since you don't seem to want the header indented, you could misuse \theoremheaderfont to include a backspace:

\documentclass{article}
\usepackage{amsmath}
\usepackage[amsmath,thmmarks]{ntheorem}
\usepackage{blindtext}
% choose your indentation here:
\theoremindent20pt 
\theoremheaderfont{\normalfont\bfseries\hspace{-\theoremindent}}
\newtheorem{proposition}{Proposition}

\begin{document}
\blindtext
\begin{proposition}
\blindtext
\end{proposition}
\blindtext
\end{document}