Two eyes, two ears, two brain hemispheres.... but only one pineal gland. What could that mean?

René Descartes used that as evidence that the pineal gland is the seat of the soul:

“My view is that this gland is the principal seat of the soul, and the place in which all our thoughts are formed. The reason I believe this is that I cannot find any part of the brain, except this, which is not double. Since we see only one thing with two eyes, and hear only one voice with two ears, and in short have never more than one thought at a time, it must necessarily be the case that the impressions which enter by the two eyes or by the two ears, and so on, unite with each other in some part of the body before being considered by the soul. Now it is impossible to find any such place in the whole head except this gland; moreover it is situated in the most suitable possible place for this purpose, in the middle of all the concavities; and it is supported and surrounded by the little branches of the carotid arteries which bring the spirits into the brain”

“Since it is the only solid part in the whole brain which is single, it must necessarily be the seat of the common sense, i.e., of thought, and consequently of the soul; for one cannot be separated from the other."

https://plato.stanford.edu/entries/pineal-gland/