6 ideas
Imagine a person created all at once, fully mature, suspended in empty air with eyes covered and limbs spread so they touch nothing and nothing touches them. They receive no sensation of their body, yet they still affirm that they exist. What they affirm, then, is something distinct from the body that is unknown to them.
Because a being can be certain it exists while being entirely unaware of its limbs or organs, the thing known cannot be the same as the thing unknown. The self grasped in that awareness must therefore be something other than the body. The soul is a real entity in its own right, not a property that arises from the arrangement of flesh.
The soul is defined as the first perfection of a natural body that has organs, and its powers stack in levels. The vegetative level handles nutrition, growth and reproduction. The animal level adds perception and voluntary motion, and the human rational level adds intellect, with each higher level including the powers below it.
Beyond the five external senses, the brain houses five inner faculties. The common sense unifies incoming perceptions, retentive imagination stores their forms, and the compositive imagination combines them. The estimative faculty grasps non-sensory meanings, and memory preserves what estimation has grasped.
A sheep flees a wolf it has never seen because it perceives hostility, a meaning that is not a color, shape or sound. The estimative faculty extracts such intentions from sensory particulars without reasoning. This explains instinctive judgment in animals and quick, non-rational appraisal in humans.
Intelligible forms are universal and indivisible, so they cannot reside in a divisible bodily organ. Intellect also does not tire from strong objects the way the eye does from bright light, and it can reflect on itself. The rational soul therefore does not rely on the body to exist and does not perish when the body does.