I believe we share existance with other beings, but our senses of sight, hearing etc. do not have sufficient deal of perception to sense them. Mostly.
Sort of like how dogs can hear sounds humans can not. That does not mean that level of sound is not existing, it is simply out of our hearing spectrum.
But these senses differ among people as well. Even for one human, during different moments such as while getting distracted or fully focusing something, you may notice certain sounds, visiuals, pain etc.or not. At certain moments our perception can be in a level to comprehend with other beings that have being referred as "spirits, ghosts". Same goes for other beings.