Sometimes an author will quote work someone else has done, but you are unable to track down the original source. In this case, both the original and the secondary source must be listed in the note and the bibliography.
If, for example, you were reading a book and the author of the book (in the example below, that would be Sarah Gwyneth Ross) made reference to the work done by another author (in the example below, that would be Astrik L. Gabriel), you would refer to the work as per the layout below.
1. Author First Name/Initial Surname [original author], Title (Place of Publication: Publisher, Year), page number, quoted in Author First Name/Initial Surname [the author of the book that refers to the thoughts/ideas of the other author]), Title (Place of Publication: Publisher, Year), page #.
2. Author Surname [original author], Title, page #.
1. Astrik L. Gabriel, "The Educational Ideas of Christine de Pisan," Journal of the History of Ideas 16, no. 1 (1995): 3-21, quoted in Sarah Gwyneth Ross, The Birth of Feminism: Women as Intellect in Renaissance Italy and England (Cambridge: Harvard University Press, 2009), 23.
2. Gabriel, "The Educational Ideas," 3-21.