Father Geiermann explains that the four internal senses—the central sense, instinct, imagination, and memory—serve as a medium between the five external senses and the intellectual faculties. He briefly explains the operation of each: “The central sense impresses the sensations of the external senses on the imagination and records them in the memory. The instinct apprehends what is fit and what unfit for the needs of animal life and arouses the appetitive faculties accordingly. The imagination forms images of natural impressions and stores them in the memory. The memory retains these images indefinitely.”
In the following, he explains that the internal senses, like the external senses, ought to be brought under the control of reason, which resides in the intellect.
“The central sense makes us conscious of the operations of the external senses.” The subjugation of the central sense to reason consists in avoiding two extremes of sense-consciousness: lethargy and sensitiveness. “A good will ought to turn instantly from any dangerous impression on the one hand, and, by distinguishing between impression and consent, have no grounds for vain fears on the other hand. We should turn as promptly from moral evil as we instinctively recoil from physical pain.” Over-sensitiveness, he notes, “retards our progress by paralyzing our energies.”
“The instinct perceives what is conducive and what is harmful to animal life. It impels man, says St. Bernard, to seek his ease, his comfort, and especially his carnal gratification.” To bring the instinct under the rule of reason, Father Geiermann suggests, among other things, that we guard against impressions that may arouse wicked suggestions and energetically subdue those we cannot avoid, that we guard against the gratification of idle curiosity, and that we strengthen ourselves by recollection and prayer. In the conflict between the internal senses and reason, he urges us to never become discouraged, but neither to imagine ourselves “immune from the assaults of the flesh.”
Quotations from Peter Geiermann, The Narrow Way (New York: Benziger, 1914).