The main objective of Paradox was to reintroduce Chrístõ as a character a few weeks before I launched the Theta Sigma series. I also wanted to play with the idea of interaction with previous incarnations, and the Seventh Doctor with his avuncular air, and Ace, the ready for anything tomboy were a perfect combination to be lost in the maze with them. Swapping the characters around, so that Ace found Doctor Number Nine and Rose was with Seven made for interesting interaction . Rose, of course, knew that the Doctor could regenerate, but the Sylvester McCoy incarnation is so very different to the Doctor she knows.
The story hangs simply on the interaction of the three versions of the same man, and three girls who all care about him in different ways, Julia looking on her Chrístõ as a sort of big brother, Ace seeing the Seventh Doctor as a warm-hearted uncle, and Rose madly in love with her Ninth Doctor, while they all try to find each other, and find a way out of the mirror maze. The three versions of The Doctor using their sonic screwdrivers to cut through the glass separating them all was not meant to be symbolic of anything. I have had readers asking if I intended a reference to the Holy Trinity. Err… no. I could just as easily have had Doctor Number Six and Mel come into the mix and have four incarnations meet. But I thought three was a manageable number. THAT’S all. There are religious analogies to be read in Doctor Who, but this isn’t one of them. The Three Musketeers, possibly. But even that would be stretching a metaphor a bit too far.
The idea of breaking the paradox by bringing something that exists more than once in the same form together is an obvious science fiction ploy, of course. Paradoxes of this sort have been mentioned on and off over the years in Doctor Who. The Blinovitch limitation effect is often quoted as a reason why two versions of the same person can’t be in the same place at once. Since we’ve had episodes like The Two Doctors, Three Doctors and Five Doctors over the years, it doesn’t seem to apply to Time Lords. The fact that their body is just a little different each time they regenerate, giving them different eye colour, hair colour, body shape, etc., explains this.
The sonic screwdriver has changed over the years. One of them was destroyed altogether in the Peter Davison episode “The Visitation”. So it is true that it couldn’t be used to break the paradox. That left only the one thing. The pendant that The Doctor gave Rose for her 21st birthday, and which his younger self gave Julia early on in their relationship. In the Theta Sigma series, he actually gives it to her for her twelfth birthday.