Theorems without begin{} and end{}

Question

My question is more a curiosity than a problem.
I notice that the following text builds a valid document. I thought the only correct way to use this theorem environment would be to write the body between '\begin{definition}' and '\end{definition}'.

Why don't I get any warnings or errors?

\documentclass{article}
\usepackage{amsthm}
\newtheorem{definition}{Definition}
\title{A reason to argue} 
\author{Grassy Knoll}
\begin{document}
\section{Introduction}
Here is a weird thing:

\definition[rain] Water that falls from the sky.  
It falls from up high.

The definition does not end, so something is wrong.
\end{document}

Answer 1

When you do

\begin{definition}

several commands are performed by \begin, followed by doing \definition.

Similarly, when LaTeX processes

\end{definition}

it executes \enddefinition and also several commands connected to \end.

If you don't use the proper \begin{...}...\end{...} LaTeX is left in an unpredictable state, because an environment is supposed to be processed inside a group, so many settings are automatically undone by the end of the group, which is provided by \end.

For instance, the font remains italic, as you perhaps discovered. And there is no vertical space after the paragraph that's supposed to be the definition text.

Output with your code

Output with the proper \begin and \end