This course will explore one of the more mature and most creative metaphysical works of St. Thomas Aquinas, the De substantiis separatis. Although an unfinished work, this treatise is complete in its philosophical consideration of “separated substances,” i.e. the intelligences or, in theological language, the angels. We will read the whole of the philosophical section of the work (which amounts nearly to the whole), attending to questions both historical and systematic as they are raised by the text.