Syntax for including a page-within-a-page

Using WP-1.5 and RunPHP plugin.
I’m looking for the syntax to include a previously written page within another page. This is a documentation project, so I’m trying to include let’s say “notes” in the “Big-Project-Page”.

I’ve tried the normal php include syntax, (variously) referencing the page slug, the page_ID, the absolute URL, none seem to work.

Whereas, creating an includes directory and referencing externally created content from there in the include statement works perfectly.

Working example(s):
<?php include ("includes/some-stuff.inc"); ?>
<?php include ("includes/some-other-stuff.php"); ?>

Non-Working example(s):
<?php include ("index.php?page_id=42"); ?>
and any other combo of the above