Logically, interfering in the past could lead to some fairly major consequences. Timelines can be completely altered, events that were supposed to happen can be changed, and an entire planet could be completely unrecognizable due to the actions of one rogue, or well-meaning time traveler.

It was interference from the Time Lords that resulted in such tragic consequences as evolution being reversed on Klist, the near destruction of Minyos, and every inhabitant on Plastrodus 14 being driven insane. After these events it was realized that something must be done. And so, in their infinite wisdom, the Time Lords decreed that a non-interference policy would go into effect. Time Lords now were forbidden to meddle in the affairs of other times, being permitted to strictly observe that which was happening.

Right. Of course this was going to work. But, you know, at least it was an attempt! And, as it turned out, it somewhat did work, as other Time Lords began to train themselves to refrain from such activities. It was much like the Prime Directive in Star Trek.

There are times when the Doctor just cannot help himself from violating that non-interference policy. He struggles between the cavalier attitude of Captain Kirk and the reserved one of Captain Picard. The Doctor has often stated that there are “fixed points in time,” such as the existence of Adolf Hitler and the death of Peter Tyler. An interference in such points could lead to disastrous consequences. This was seen when Pete was kept alive and caused the Reapers to suddenly find Earth to be interesting, so at least those specific cases have a built in deterrent.

There are times, however, when the Doctor will interfere with time to his hearts’ content, fixed point or not. Remember Pompeii and how everyone was supposed to perish due to the eruption of Mount Vesuvius? Well, the Doctor, albeit after the pleading of Donna, saved Lobus Caecilius and his family. In fact, the Doctor himself was an agent of the Time Lords to interfere elsewhere in the universe and across time, despite that official, and supposedly overriding, policy.

In all seriousness, where would Doctor Who be if the Doctor only observed? He has to interfere – it is a part of who the Doctor is, and it may well be wired into his Time Lord DNA. He is The Doctor, a title selected because he tries to fix the ills of the universe. This conscious decision to interfere is exactly what makes him who he is.

Yes, there is a non-interference policy for the Time Lords. Fortunately, as the Last of the Time Lords, the Doctor can determine when it should be enforced. The answer, quite obviously, is almost never.