1. Assume, for simplicity, that everyone has two eyes that they see with (apologies, then, to pirates and cyclopses).
2. Assume, for simplicity, that everyone has two eyes that are seen (again, apologies).
3. Eyes that are seen are objective, that is, everyone else can see them.
4. Eyes that you see with are subjective, that is, only you can see with them.
5. Eyes that are seen are spatial (they have locations and shapes) and this is readily observable.
6. Eyes that you see with are nonspatial (they have neither locations nor shapes) since they occupy no position or amount of the visual field.
7. Eyes that are seen with cannot be identical to eyes that are seen since an objective spatial thing cannot be identical to a subjective nonspatial thing.
Therefore, everyone has four eyes.