SRP & "axis of change"?

Posted by lance on Stack Overflow See other posts from Stack Overflow or by lance
Published on 2010-06-01T19:29:56Z Indexed on 2010/06/01 19:33 UTC
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reading

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principles

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single-responsibility

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wording

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"?

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