Perception is inside your head, yes, and vision too, but that is not what I meant by the remark. I just meant that there is no trick of geometry that causes the misperception. If you look to your left, the left rail looks straight. If you look to your right, the right rail looks straight. If I look at the moon and the sun, the line between them looks straight, and perpendicular to the terminator. Things in the scene may cause an illusion--similar to the optical illusions that were brought up before, straight lines drawn across a field of curved lines sort of thing that create an illusion that the straight lines are not straight.