citations - Citing an unknown primary source?

I'm using an unknown primary source quote in my document. Harvey references it in this book: https://books.google.co.uk/books?id=a2hnAAAAMAAJ&redir_esc=y

However does not cite it. How do I cite it in my document? I know the rough date.

Thanks

Answer

Only cite sources you've actually seen. In this case, it sounds like you have a secondary source (your link) that quotes from and does not cite a primary source. All you can say with certainty is that this secondary source says that primary source says what it does.

The way you handle this in your document is to say something like "according to (secondary source), so-and-so said X in approximately (date)". You then cite your secondary source.

You can increase the veracity of the claim about the primary source by finding other secondary sources that say the same thing. But if you haven't seen the primary source yourself, don't cite it. It will come across as dishonest, and if it turns out that your secondary source is wrong about the primary source, it will call into question your credibility for anything else you've cited.