Many, many moons ago, I was a scientist. Oh yes, I took a degree in astronomy. For a number of complex reasons, that didn't work out and now I'm a local government officer. But hey, those are the breaks... Nevertheless, I've always maintained an interest in cosmology, especially the more cutting edge ideas of quantum mechanics. I'm absolutely fascinated by it, and sometimes I even like to pretend that I can understand it all...
Why am I telling you this now? Well, believe it or not, it has a very great relevance to the problems of Doctor Who continuity. Seriously. The Many Worlds Interpretation is an incredible piece of cosmological thinking - I really can't describe it here, but here's Wikipedia's page on it, which gives a very good overview. Alternatively, you might like to try this page. But here's my (possibly inaccurate) layman's summary of it:
The Many Worlds Interpretation (MWI) posits that all possible outcomes to every single event exist simultaneously within the universe, each within their own mutually unobservable "world". What this means is that history ceases to be a linear progression, a single (four-dimensional) path through the space-time continuum. Rather, it's more like an ever-branching tree, where every possible event is to be found somewhere along one of its branches.
So what does this mean for Doctor Who continuity? Well, we've all heard the arguments, such as "the comics contradict the tv series, so they can't be canon"; or "Richard E Grant and Christopher Eccleston can't both be the ninth Doctor, so Shalka isn't canon". Leaving aside for now that canon is a dirty word (I'll come back to that at a later date), you can see how MWI slices through such arguments. It's perfectly possible for contradictory outcomes to exist within the same space-time continuum. You don't need to resort to putting books and audios into parallel universes or anything like that. The key is that phrase "mutually unobservable". Once each possible outcome has split into its own "world", it can't interfere or interact with any of the other "worlds". In any instant, you can only observe the version of the space-time continuum you happen to be in at that instant. Every time the Doctor steps out of the Tardis, he changes the conditions of the universe around him. At any moment, only some of his past adventures might actually have happened to him.
On a prosaic level, it accounts for how both The Paradise of Death and Invasion of the Dinosaurs can follow on from The Time Warrior; or how The Infinity Doctors can feature a short-haired eighth Doctor living on Gallifrey. It can even account for how the fifth Doctor doesn't remember having lived through the events of The Five Doctors four times over already.
In a series whose central character can travel in time, and where history has been shown to change at the drop of a hat, it seems crazy to want to order the universe into a single, coherent, linear timeline. And here's real science to say that actually time doesn't work like that.
Not so long ago, I was nearly hit by a car. I was using a pelican crossing, and the man was green - but this car came out of nowhere, right through the red light and didn't even try to slow down. Some instinct made me pull myself out of the way. Weirdly though, I didn't feel shaken up or anything. I was really calm about it, and I found myself rationalizing the incident according to MWI. I realized that in a different part of the space-time continuum, my broken body was flying through the air and smashing down onto the surface of the road. But not in this "world". I was alive because I was in that quantum reality where the car missed me. You can only observe the part of the space-time continuum you happen to be in at any instant - and obviously, I couldn't observe a reality in which I was dead. (Go and do a web search for "quantum suicide" if you want some really extreme thinking along these lines...)
So there you are. Somewhere in the universe, I stuck with the astronomy and became a scientist.