Heart of the TARDIS

The Heart of the TARDIS is the ship's soul. The connection to the Eye of Harmony is unclear. It is located beneath the central console, as configured by the Ninth Doctor. On one occasion it opened as if by itself (DW: Boom Town), and on another it seemed to take a great amount of strength to pull open the TARDIS console and reveal the Heart, such as a rift in space and time, or a large truck. (DW: The Parting of the Ways)

The presence of the Doctor in the first case, but not in the second, may example this inconsistency.

When Blon Slitheen looked into the Heart she regressed back to a Raxacoricofallapatorian egg. (DW: Boom Town)

When Rose Tyler did so she could telepathically operate the TARDIS, sending it to Satellite Five and inheriting god-like powers. In the process she also got infused with vortex energy. To save her life, the Doctor took the energy upon himself. (DW: The Parting of the Ways)