Table with a diagonal line that crosses cells

Here is one way of doing it, using the graphicx package:

\documentclass[10pt,letterpaper]{article}
 \usepackage{graphicx}
\begin{document}
\begin{table}[]
    \centering
    \caption{My caption}
    \label{my-label}
    \begin{tabular}{|l|l|l|l|l|l|l|}
        \hline
        \multicolumn{1}{|c|}{CONCEPT} & \multicolumn{6}{c|}%
{COMPREHENSIVE NOTES\Large\strut}                                \\ \hline
        Man                            & Substance & Material & Living           &     Sentient & Rational & Man \\ \hline
        Animal                         & Substance & Material & Living           &     Sentient & Brutes   & Man \\ \hline
        Organism                       & Substance & Material & Inanimate Bodies &     Plants   & Brutes   & Man \\ \hline
        Body                           & Substance & Material & Inanimate Bodies &     Plants   & Brutes   & Man \\ \hline
        Substance                      & Substance & Spirit   & Inanimate Bodies &     Plants   & Brutes   & Man \\ \hline
        \multicolumn{1}{|c|}{}         & \multicolumn{6}{c|}%
{EXTENSIVE SUBJECTS\Large\strut}                                 \\ \hline
    \end{tabular}


\vskip-1.17in\rotatebox{14}{\hskip1.5in\vtop{\hsize=3.75in\rule{3.28in}{1pt}}}



\end{table} 

and adjust the dimensions to fit your needs. I added a \Large\strut to adjust the cell padding for the top and bottom cells.

With tikz

\documentclass[a4paper]{article}
\usepackage{tikz}

\newcommand{\tikzmark}[2][]{\tikz[remember picture, overlay]\node[#1,inner sep=0pt,xshift=-\tabcolsep](#2){};\ignorespaces}
\newcommand{\tm}[2][]{\tikzmark[yshift=#1]{#2}}

\begin{document}
\begin{table}[]
\centering
\caption{My caption}
\label{my-label}
\begin{tabular}{|l|l|l|l|l|l|l|}
\hline
\multicolumn{1}{|c|}{CONCEPT}& \multicolumn{6}{c|}{COMPREHENSIVE NOTES}                                                       \\ \hline
Man                          & Substance & Material               & Living           & Sentient & Rational &\tm[1.8ex]{A} Man \\ \hline
Animal                       & Substance & Material               & Living           & Sentient & Brutes   & Man              \\ \hline
Organism                     & Substance & Material               & Inanimate Bodies & Plants   & Brutes   & Man              \\ \hline
Body                         & Substance & Material               & Inanimate Bodies & Plants   & Brutes   & Man              \\ \hline
Substance                    & Substance & \tm[-0.9ex]{B} Spirits & Inanimate Bodies & Plants   & Brutes   & Man              \\ \hline
\multicolumn{1}{|c|}{}       & \multicolumn{6}{c|}{EXTENSIVE SUBJECTS}                                                        \\ \hline
\end{tabular}
\tikz[remember picture, overlay]\draw[thick] (A)--(B);
\end{table}
\end{document}