It is impossible to manifest any psychic ability without first stimulating the chakras. Many people will deny this, above, and claim they have never done any chakra, energy work, but still have psychic abilities. There are many ways to develop yourself, agreed, but they all, directly or indirectly, stimulate the chakra system. And let's not forget natural ability. Many people are born with naturally active chakras and hence, natural psychic ability.
Mediums are people who exhibit psychic abilities when aided by a nonphysical spirit entity.
This entity stimulates the chakras of the medium directly, by melding with the medium and causing psychic abilities, clairvoyance, channelling, healing, production of ectoplasm etc to manifest through the medium. This is why they are called Mediums or channels, i.e., they have the ability to become a passive vehicle for a spirit entity to affect, or communicate with, the physical world. You do not need a spirit entity to develop and use, a psychic ability. If you learn to control your own chakras and energy, you can do these things on your own, with no spirit entity involved, and without the inherent risk involved with that method. Raising Sit, or lie down, do the relaxation exercise and clear you mind with breath awareness.