How did Thomas Aquinas explain ideas about things that cannot be seen?

St. Thomas Aquinas’s epistemology is rooted in the principle that human knowledge begins with sense experience. For Aquinas, the mind gains knowledge when the active intellect abstracts essential concepts from images provided by the senses.1

Regarding things that cannot be directly seen—such as general truths or the nature of objects—Aquinas explained that the intellect isolates the "essential" elements of an object from the specific, observable details of a particular instance. By abstracting these essentials, the mind forms a "species," which acts as the apparatus of knowledge. This allows the intellect to grasp the general nature or "form" of a thing, rather than just its physical appearance.1

Furthermore, Aquinas maintained that while human knowledge is based on sense experience, the mind engages in reflective activity to understand deeper realities. He argued that human nature is a union of soul and matter, and through the intellect's ability to abstract and reflect, humans can attain knowledge that transcends immediate sensory input, ultimately aiming toward the understanding of existence and, in a theological context, the knowledge of God.21