What 'hard to vary' really is

To the extent a system of thought exhibits qualities such as internal
consistency, rigour and emphasis on 'consequences' then hard-to-varyness
will emerge as a straight consequence of the fact existing principles
will act as constraints on subsequent principles.

It isn't a property of individual explanations. It isn't a quality of
individual explanations. It is a quality of the relationships between
principles.

Nor is it an indication of how difficult or close to reality the
explanations are getting. This is because a system of thought creates
its own reality, the constraints on subsequent explanations are decided
entirely by the legacy of former explanations.

The substance of hard to varyness most closely reflects how easily new
principles are added to the system. In the case of the PopperDeutsch
system, new principles are added immensely easily and that is because
there all forms of criticism involving external systems, such as
scientific evidence, common intuition, real world events, history, and
so on, the system has a veto over.

So the major constraint on new explanations is that it must be
consistent with what is already there. The major potential for criticism
of new explantions come entirely from within the fold of popperianism,
which means not of the kind that will be challenging the 'constraints'
on that new explanation, but only the extent it conforms to those
constraints.

Principles like the jump to universalism are by definition actually
described as "easy to happen", and for this reason new instances of
jumps to universality in the system act as incredible constraints which
generate immense forces of 'hard to varyness' but now the meaning of
that because negative.

It's worth noting that 'hard to varyness' in Science might be associated
with brilliant difficult to find theories, but the reason is because the
explanations have to cover so many bases and be consistent with so many
things, all of which can be thrown out by a whole variety of kinds of
criticism. They are not protected the way popperianDeutschian ideas are.

In other systems, hard-to-varyness can be a bad thing. It's totally
defined by how easily principles are added. IN metals, hard to vary
means brittle. It can mean brittle for you too.