But the pineal gland has no physiological counterpart in any other part of the brain, so Descartes considered it to be the locus of mind-brain interaction. The soul is created and united to the body by God, and it is separated from the body by God at death. Since a soul has no parts, it is indivisible. But since the soul has no parts the mind is incorruptible. Our souls survive the death and corruption of our bodies.
Souls are unique to human beings.
Non-human animals, no matter how complex and large their brains do not have souls. If animals had souls, they would be immortal and capable of thinking, self-consciousness, and language use, just as we are. But animals cannot do these things. Thus, Descartes held that despite their many abilities, animals are nothing more than complex biological robots.