SRP & “axis of change”?

Question

I'm reading Bob Martin's principles of OOD, specifically the SRP text, and I understand the spirit of what it's saying pretty well, but I don't quite understand a particular phrasing, from page 2 of the link (page 150 of the book):

I paraphrase:

It is important to separate these two responsibilities into separate classes because each responsibility is an axis of change.

What exactly is meant here by "axis of change"?

Answer 1

Changes occur around the axis and that suggest that each responsibility acts as a central point for the class's existence. You want a class to have a single reason to exist such that changes are easier to manage.