瑜伽師地論略纂 5.75

5.75

有學者四空差別中，「又修行者」者，謂諸凡聖有學，由彼果空。　「或時作意思惟外空」等者，此說由希彼果空故，思惟外、內二境為空；或由觀察所執空故，思惟內、外二種俱空。

Among the distinctions of the four kinds of emptiness of those in training, "further, a practitioner" refers to ordinary beings and noble persons who are in training, on account of the emptiness that is its fruit. "At times with attention reflects on external emptiness..." and so forth: this states that out of aspiring to the emptiness that is its fruit, one reflects on both external and internal objects as empty; or, on account of observing the emptiness of the imagined, one reflects on both internal and external as empty.