The soul is the form of the body. Its' existence can be known with certainty.
The soul is the form of the body.In terms of Aristotle's four causes:
Body: material cause of a living organism.Soul: formal cause of a living organism.
Soul: efficient cause of a living organism. (All substances that exist by nature are self-moving; living things are natural substances; therefore, living things are self-moving. To accommodate for plants, self-movement is not limited in meaning to change of place (locomotion), but includes change of quality (alteration) and change of quantity (growth and decay) (De Anima, 415b23-28)).
Soul: final cause of a living organism. (The soul is that in whose interest everything is, or is done, in a living organism (Cf. Ibid., 415b20). For example, wisdom is excellence of reason; reason is a faculty of the soul; therefore, wisdom is excellence of soul.)