In other threads there has been mention of a 3d retina for a 4d being. I'm wondering if this has to be so. Vision evolution in a 4d environment may begin with simple vision in one direction only. This would be an x/y 2d plane of vision.
The problem is that this vision would be looking at 4 spatial dimensions - two more than itself, but it could only see the 2d boundary of all that is around it, including the body of the being it is within. It would see what it would think is 3d space around itself, even though it would really be 4d.
Rotations in 4d take place about a plane, but if that 4d being can only see the 2d boundary of such an object's rotation, wouldn't it think only a rotation about an axis is taking place?