A primary source reports original content.
A secondary source refers to content first reported in another source.
It is preferred to cite primary sources. You should cite secondary sources when:
To learn more about citing secondary sources, see p. 258 of the manual.
To cite a secondary source:
Say you read an article by Knipe & Horowitz (2018) in which Truslow (2009) was cited. You are unable to read Truslow's work yourself. The reference list would only include the secondary source by Knipe & Horowitz (2018). The in-text citation would look as it does below:
(Truslow, 2009, as cited in Knipe & Horowitz, 2018)
If the year of the primary source is unknown, omit it from the in-text citation.
Newton's diary (as cited in Hale, 1912)