Ennead IV.4.30–45 and IV.5 retrieves the unity in this last section of Plotinus’ treatise on Problems concerning the Soul. Combining translation with commentary, Gurtler enhances both the accuracy of the translation and the recovery of Plotinus’ often unsuspected originality. This is especially true for IV.5, where previous translations fail to convey the concise nature of his argument against both the Aristotelian and Platonic theories of vision.

Plato and Aristotle each claim that vision depends on the light between the eye and the object, but Plotinus presents evidence that this is not the case and develops a novel theory of light as a second activity that moves from source to object directly, even arguing that color is in the light itself rather than merely a quality of the object. This theory of vision, in turn, depends on the nature of sympathy developed especially in IV.4.30–45, where Plotinus shows how action at a distance is both possible and necessary for the proper unity in diversity of the sensible cosmos.

Plotinus was a Platonist, committed to expounding the doctrines put forward by Plato some seven centuries earlier. He was born and educated in Egypt, where he studied the teachings of Plato under the guidance of Ammonius Saccas. He came to Rome in 244 CE and built up a circle of followers devoted to studying Plato through Plato's own works and those of philosophers, both Platonist and non-Platonist, of the intervening centuries. From his fiftieth year Plotinus himself wrote down, in Greek, the findings of the seminars, and these writings were later edited by one of his pupils, Porphyry, and published in six groups of nine treatises entitled the Enneads (from the Greek word for nine – ennea).