Some say that cats have a sixth sense, that they can see and hear things that we can't.  We know that their hearing is much more sensitive than ours.  PetMD reports that cats (and some other animals) see in ultraviolet light.  That ability opens up a whole different world than the one we see.  Our lens blocks UV from reaching the retina.  So, cats might see fascinating patterns on birds wings, or leaves, or staircases. They see things that our eyes can't detect.  Admittedly, it is at times unnerving to see your cat intently watching or responding to something that you can't see or hear.  But that only adds to their magic and mystery.