The Doctor's mind

The Doctor's mind was the inner consciousness within the Doctor's brain.

History
During the Time Paradox Incident, the Daleks used a Mind Analysis Machine to forcibly ascertain the identity of the Third Doctor, projecting the familiar images of the First and Second Doctors on their screen. (PROSE: Doctor Who and the Day of the Daleks, TV: Day of the Daleks)

The Third Doctor once saw images of the Master inside his mind during a nightmare. (TV: The Time Monster)

In a dream, the Fourth Doctor encountered his previous incarnations in a heavenly garden, which made him aware that his own regeneration was on the horizon. (PROSE. Into the Silent Land)

During his regeneration, the Fifth Doctor was trapped in a dreamscape by, exploiting a residual connection of their minds from their connections with Kamelion, in an attempt to stop the process. The Doctor subconciously reached out telepathically to Nyssa who helped him break free and complete the regeneration. (AUDIO: Winter)

During the Seventh Doctor's battle with the Timewyrm, Ace entered the Doctor's mind, encountering various past Doctors, including the First, Third and Fourth Doctors. She discovered the Fifth Doctor had become symbolic of the Doctor's conscience and freed him from the chains the current Doctor's persona had confined him in. (PROSE: Timewyrm: Revelation)

Bernice Summerfield entered the Doctor's mind after the Seventh Doctor was possessed by the Scourge. She tried to help him resist their despairing influence and caught glimpses of his past selves and possible future self. (AUDIO: The Shadow of the Scourge)

Fearing his potential to become the Valeyard, the Seventh Doctor buried memory of his predecessor deep within his mind. (PROSE: Head Games) After being buried alive, the Seventh Doctor came to recognise no-one deserved to be trapped this way and released him. (PROSE: The Room With No Doors)

When the Eighth Doctor was possessed by Zagreus and stabbed by Charley Pollard, he met the mental manifestations of his fifth, sixth and seventh incarnations in his mind when he nearly allowed himself to die, subsequently rallying himself with their aid to restore himself and retake control of the anti-time energies. (AUDIO: Zagreus)

The Eighth Doctor tricked the Kro'ka into entering his mind, where he was able to briefly hold him prisoner and interrogate him for information on the whereabouts of his TARDIS. (AUDIO: Caerdroia)

Mr Clever attempted to take over the Eleventh Doctor's mind when trying to convert him into a Cyberman. The mind was subsequently split into two halves; 49.881% in the Doctor's control and 49.881% under Mr Clever's influence. The two sides differentiated, with Mr Clever's half coloured silver and blue with circuitry scattered around, while the Doctor's was orange with Gallifreyan symbols covering it. (TV: Nightmare in Silver)

Within the mind, the Doctor could apply a mental block to prevent intruders from seeing things he did not want them to see. (TV: New Earth, Nightmare in Silver) However, if the Doctor was purposefully connecting with another, namely Jeanne-Antoinette Poisson with the Tenth Doctor, they could see into his memories, so long as the Doctor maintained the connection. (TV: The Girl in the Fireplace) Once, the Eleventh Doctor rapidly connected his mind with Craig Owens' to speedily share information, although the process resulted in a mild headache. (TV: The Lodger)

Regeneration energy could also transfer the knowledge within the Doctor's mind to another. Donna Noble gained the Tenth Doctor's mind by touching his severed hand while it was infused with regeneration energy from a recent regeneration. However, given she was human, the Tenth Doctor had to remove all memory of himself from her soon after to ensure her mind did not burn from trying to possess a Time Lord consciousness. (TV: Journey's End)

The Twelfth Doctor could willingly enter his mind to quickly deduce problems when under extreme duress. While there, his mind took the image of his TARDIS while he worked out solutions. Given how quickly his mind processed information, what seemed like minutes to him was mere seconds outside his mind. (TV: Heaven Sent)

Acter being forced to regenerate by, the Thirteenth Doctor passed into her mind but was halted at the Edge of Existence by manifestations of her past selves, calling themselves the Guardians of the Edge, to stop her going past the point of no return. This enabled Yaz and Vinder to restore her via degeneration. (TV: The Power of the Doctor)

Behind the scenes

 * The use of the Mind Analysis Machine in Day of the Daleks was parodied in an installment of Doctor Whoah!, where the Doctor is flustered as his mind wonders toward a romanticised image of the Brigadier in his underpants, and he tries to convince the Daleks that it is nothing more than "a brilliant fiction" he concocted to distract them.