Why do class methods always need 'self' as the first parameter?

[T. Goodchild]

I’m new to Python, and I love it.  The philosophy of the language (and
of the community as a whole) is beautiful to me.

But one of the things that bugs me is the requirement that all class
methods have 'self' as their first parameter.  On a gut level, to me
this seems to be at odds with Python’s dedication to simplicity.

For example, consider Python’s indent-sensitive syntax.  Although
other languages didn’t use indentation to specify scope, programmers
always used indentation anyways.  Making indentation took a common
practice, made it a rule, and the result was a significantly improved
signal-to-noise ratio in the readability of Python code.

So why is 'self' necessary on class methods?  It seems to me that the
most common practice is that class methods *almost always* operate on
the instance that called them.  It would make more sense to me if this
was assumed by default, and for "static" methods (methods that are
part of a class, but never associated with a specific instance) to be
labelled instead.

Just curious about the rationale behind this part of the language.