Canonical way to typeset proper descriptions in the style of Principia

I want to typeset "the x such that Phi(x)" thus:

My MWE:

\documentclass{article}
\usepackage{graphics}
\begin{document}
\newcommand{\mthe}{\rotatebox[origin=c]{180}{\iota}}
$(\mthe x)(\Phi x)$
\end{document}

This appears to be typographically adequate. However, I believe I read somewhere that using graphics transformations to achieve something like this is not robust. Can anyone suggest a better way?

• This symbol is Unicode 2129, "turned Greek small letter iota". I don't remember whether this got into the stix font, nor whether (if it did), the orientation is upright or italic. But that's a place to look. And if you can provide a published citation, in context, it can be submitted to Unicode for consideration as a math symbol. Looks to me like a good candidate. – barbara beeton Jul 13 at 13:20
• @barbarabeeton Many thanks, barbara. It's in the stix fonts as \turnediota if you'd like to add an answer, I'll be delighted (and horoured!) to accept it. – Brent.Longborough Jul 14 at 8:17
• @Brent.Longborough I'm always of the opinion that being nice is something truly extraordinary. – Sebastiano Jul 14 at 15:06