modular.tex

Create a blank file called modular.tex and insert the following (preferably with no whitespace before or after):

\def\mainfile#1{
  \ifx\processingmainfile\undefined
    \def\processingmainfile{processingmainfile}
    \message{TeX run on an included file, jumping to the main file (#1)}
    \input #1
  \fi
}

Include the file in all your subordinate files and reference the main file like this:

\input modular
\mainfile{thesis.tex}