Warning: Spoilers ahead for Doctor Who season 15, episode 5, "The Story and the Engine."Doctor Who regeneration sequences.

Going back a few years earlier than that, Chris Chibnall's Timeless Child storyline revealed that regeneration is an ability that originated with the Doctor. Furthermore, Chibnall's big twist made it so that the Doctor has unlimited regenerations, whereas the show's main character had long been established as having a limited number of times he could renew himself. Similar alterations have permeated the franchise depending on the era and what was required at the time. Fifteen's description of a post-regeneration life doesn't retcon the process, but it does add a fascinating spin on what it feels like.

Doctor Who Season 15 Suggests Previous Regenerations Linger Inside The Doctor's Mind

Every Doctor is still there, speaking with Fifteen & each other

Fifteen's interaction with the Story Engine in Doctor Who season 15, episode 5, results in the Doctor's many lives being fed into the machine. The sheer length of the Doctor's lifespan is enough to overwhelm the Barber's creation, but before it blows up, it shows a myriad of clips from the Time Lord's existence. During this scene, Fifteen tells the Barber about what it feels like to have lived so many lives, and makes the surprising ission that he's aware of all his past personalities lingering in his brain.

"My body is like a barbershop. All of them, inside, telling their stories, bickering. I will not fail them."

- The Fifteenth Doctor in "The Story and the Engine."

There is an endless debate among Doctor Who fans when it comes to what regeneration means for a version of the Doctor. One argument is that the character dies, and a new person steps in to take over, who just so happens to inherit the cumulative memories of their predecessor. David Tennant's Tenth Doctor certainly felt this way and itted to his fear of regeneration near the end of his run. Another train of thought is that the divide between Doctors isn't quite so solid, and that it is the same person coming back each time with a new face.

Fifteen's view of the process is one I had never really considered, and I find it a fascinating reevaluation of regeneration.

Fifteen's view of the process is one I had never really considered, and I find it a fascinating reevaluation of regeneration. The implication is that all the previous Doctors are always with their active successors, available to communicate with and help them whenever required. Of course, Gatwa's Doctor may only have been speaking figuratively, and perhaps he simply views all his predecessors as separate entities or ancestors rather than past versions of himself. Either way, it's a refreshing take on a very old concept.

Why So Many Of The Doctor's Regenerations Don't Get Along

Hearing that the other Doctors are often "bickering" in Fifteen's mind is no surprise

Even though every version of the Doctor has many of the same core values and personality traits, there's also an awful lot of wiggle room when it comes to how different they can all be from one another. Every actor to lead Doctor Who has redefined the role in their own special way, resulting in many versions of the character who have their own standout traits that set them apart. As a result, there are various versions of the Doctor who are just too different from one another to be able to get along smoothly.

Multi-Doctor stories quite often prove that this is the case. For example, John Hurt's stern War Doctor disapproves of the playful streak he sees from David Tennant's Tenth Doctor and Matt Smith's Eleventh in 2013's "The Day of the Doctor." Inversely, David Bradley's First Doctor and Peter Capaldi's Twelfth are pretty similar in their respective dispositions, but that leads to an understandable level of friction during their team-up in 2017's "Twice Upon a Time." In short, Doctor Who's main character collaborating with a past or future version is almost guaranteed to result in sparks flying.