Align left side of table with \parindent

Question

I am using the tabularx environment to present formal arguments, and at the moment the left column is aligned with the left margin of the text and the right column is aligned with the right margin. For instance:

How can I get the left column to be aligned with the paragraph indent, but the right column to stay aligned with the right margin? I've tried using the changepage package and the \adjustwidth command but that doesn't seem to be helping, nor does adding \hspace{\parindent} in various places. I'm sure there's a really simple way to do it but I don't know how.

Here's a MWE:

\documentclass[12pt]{article}

\usepackage{tabularx} %tables
\usepackage{tabulary} %tables
\usepackage{lipsum}
\usepackage{upgreek} %greek letters
\usepackage{amssymb} %modal operators

\begin{document}

\lipsum[1-2]

\begin{table}[h!]
    \centering
    \setlength{\extrarowheight}{3pt}
    \begin{tabularx}{\textwidth}{@{}rXr@{}}
        (1) & $\varnothing \vdash \pi \leftrightarrow val(\ulcorner \pi \urcorner, \ulcorner \bot \urcorner)$ & ($\pi$ def.)\\
        (2) & $\pi \vdash \pi$ & (Assumption)\\
        (3) & $\pi \vdash val(\ulcorner \pi \urcorner, \ulcorner \bot \urcorner)$ & (1,2, MP)\\
        (4) & $\pi \vdash \bot$ & (2,3, \textit{V-Detach})\\
        (5) & $\varnothing \vdash val(\ulcorner \pi \urcorner, \ulcorner \bot \urcorner)$ & (4, \textit{V-Schema})\\
        (6) & $\varnothing \vdash \pi$ & (1,5, MP)\\
        (7) & $\varnothing \vdash \bot$ & (5,6, \textit{V-Detach})\\
    \end{tabularx}
\end{table}

\lipsum[3-4]


\end{document}

Thanks!

Answer 1

The default value of \parindent in the article document class is 1.5em. To achieve your formatting objective, it therefore suffices to change

\begin{tabularx}{\textwidth}{@{} rXr @{}}

to

\begin{tabularx}{\textwidth}{@{\hspace{1.5em}} rXr @{}}

Since the overall usable width of the tabularx environment is decreased by 1.5em, LaTeX will automatically decrease the width of the X colum by that amount in order to assure that it will continue to fit inside the textblock.

---

Addendum: In case you don't happen to know the value of \parindent, you could execute, say,

\newlength\origparindent
\setlength{\origparindent}{\parindent}

sometime before starting the table environment and replace

\begin{tabularx}{\textwidth}{@{\hspace{1.5em}} rXr @{}}

with

\begin{tabularx}{\textwidth}{@{\hspace{\origparindent}} rXr @{}}

Answer 2

You don't need to know the value of \parindent, because TeX knows it.

Also you don't want a table environment, because it's a floating object and you quite likely want the material there, not somewhere else.

You can use an enumerate environment, properly set up with the help of enumitem. Pushing something to the right margin is easy with \hfill, so long as the preceding material allows for the placement, more precisely it wouldn't overlap the label.

\documentclass[12pt]{article}
\usepackage{amsmath}
\usepackage{lipsum}
\usepackage{upgreek} %greek letters
\usepackage{amssymb} %modal operators
\usepackage{enumitem}

\newenvironment{sidecond}
 {\begin{enumerate}[
    label=(\arabic*),
    leftmargin=\dimexpr\parindent+\labelsep,
    labelwidth=0pt,
    align=left
]}
{\end{enumerate}}

\begin{document}

\lipsum[2][1-5]

\begin{sidecond}
\item $\varnothing \vdash \pi \leftrightarrow 
       \mathit{val}(\ulcorner \pi \urcorner, \ulcorner \bot \urcorner)$\hfill ($\pi$ def.)

\item $\pi \vdash \pi$\hfill (Assumption)

\item $\pi \vdash \mathit{val}(\ulcorner \pi \urcorner, \ulcorner \bot \urcorner)$\hfill (1,2, MP)

\item $\pi \vdash \bot$\hfill (2,3, \textit{V-Detach})

\item $\varnothing \vdash \mathit{val}(\ulcorner \pi \urcorner, \ulcorner \bot \urcorner)$\hfill
      (4, \textit{V-Schema})

\item $\varnothing \vdash \pi$\hfill (1,5, MP)

\item $\varnothing \vdash \bot$\hfill (5,6, \textit{V-Detach})
\end{sidecond}

\lipsum[4][1-3]


\end{document}

A couple of final notes. I used \mathit{val}, because otherwise the letters are too spaced out (because they denote the product of three variables). I left \varnothing even if I abhor it for denoting the empty set.