# Square of oppositions diagram with circle inside

I'm trying to draw the following image (without the scribbly line):

So far, I can come up with the following code:

            \documentclass{article}

\usepackage{tikz}

\begin{tikzpicture}[
corner/.style={font=\bfseries\large\sffamily},
arrow/.style={->,>=stealth,thick},
label/.style={font=\small\sffamily,fill=white,midway},
contra/.style={thick}
]
\node[corner] (E) at (2,1)  {$^{+}\Sigma^{-}$};
\node[corner] (O) at (2,-1)  {$^{+}\Psi^{-}$};
\node[corner] (I) at (-2,-1)  {$^{+}\Theta^{-}$};
\node[corner] (A) at (-2,1)  {$^{+}\Delta^{-}$};

\draw[contra] (A) -- (I);
\draw[contra] (E) -- (O);
\draw[contra, dotted] (A) -- (O);
\draw[contra, dotted] (I) -- (E);
\draw[contra] (A) -- (E);
\draw[contra] (I) -- (O);
\end{tikzpicture}

\end{document}


I'm not sure how to put the circle diagram in the middle of the box and have dotted lines (with "R" above them) from the four corners of the square extending to it.

Does anyone have an idea of how to do this?

You can use an ellipse node with a white background like this:

\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{shapes}
\begin{document}
\begin{tikzpicture}[
corner/.style={font=\bfseries\large\sffamily},
arrow/.style={->,>=stealth,thick},
label/.style={font=\small\sffamily,fill=white,midway},
contra/.style={thick}
]
\node[corner] (E) at (2,1)  {$^{+}\Sigma^{-}$};
\node[corner] (O) at (2,-1)  {$^{+}\Psi^{-}$};
\node[corner] (I) at (-2,-1)  {$^{+}\Theta^{-}$};
\node[corner] (A) at (-2,1)  {$^{+}\Delta^{-}$};
\draw[contra] (A) -- (I);
\draw[contra] (E) -- (O);
\draw[contra, dotted] (A) -- (O);
\draw[contra, dotted] (I) -- (E);
\draw[contra] (A) -- (E);
\draw[contra] (I) -- (O);
\node[draw, fill=white, ellipse] at (0,0) {$\Gamma+\Pi$} node[above left=0.3cm] {R};
\end{tikzpicture}
\end{document}