Laws of Time

The Laws of Time were a set of guidelines and laws guiding what a time traveller could do. As opposed to physical laws, which were observations of constants of behavior found in nature, like the laws of motion or gravitation, the Laws of Time were rules put in place to prevent massive changes to the primary timeline by time travellers.

They traditionally prevented Gallifrey's 'present' from interacting with its subjective past or future. (NA: Lungbarrow).

It was unknown how those law were enforced, or what was the exact punishment for breaking them. It was possible the effects of breaking them — such as the destruction of the causal nexus or the deterioration of the Web of Time were serious enough that they would themselves serve as a deterrent.

The Doctor stated that he was "...Defender of the Laws of Time" in his seventh persona and eighth persona. (DW: Remembrance of the Daleks, EDA: Vampire Science)

The Laws of Time became weakened during the Second War in Heaven so that future events filtered back to influence interact with events prior to the war's outbreak. (EDA: Alien Bodies, The Taking of Planet 5)

Fixed points in time
The Laws of Time were connected to the concept of "fixed points" in time — events and/or individuals who have such a long-standing impact on the timeline that no one, not even Time Lords, were allowed to interfere with their natural progression. The Doctor, while free to interfere in alien invasions and save planets in most cases, cannot interfere/interact with these fixed points. Examples included Jack Harkness after his rejuvenation by Rose Tyler (DW: Utopia (although the Doctor nonetheless shared several adventures with him), the destruction of Pompeii by the Vesuvius volcano (DW: The Fires of Pompeii, and the death of explorer Adelaide Brooke (DW: The Waters of Mars).

The case of Brooke marks one of the only times the Doctor intentionally interfered with a fixed point, under the rationale that, as the last surviving Time Lord, the Laws of Time were his to command. Ultimately, he was unable to prevent Brooke from committing suicide, thereby allowing the timeline to unfold, with only a few minor changes. (DW: The Waters of Mars)

Exactly what constituted a fixed point in time was unclear. For example, the birth of Earth religious leader Jesus Christ was a major event in the cultural evolution of the planet Earth, so much so its common calender was dated from his estimated year of birth, and innumerable aspects of human civilization related to worship of the Son of God. Yet the Doctor once claimed to have influenced the birthplace of Jesus, who was born in a barn due to the inn in Bethlehem having no rooms; the Doctor said he'd gotten the last room at the inn. (DW: Voyage of the Damned). However, much like the eruptions of Vesuvius, it may have been simply a case of the Doctor experiencing a predestination paradox and being part of the natural progression of events.

Specific Laws of Time
The Time Lords codified the rules regarding time travel into a set of enumerated "Laws of Time". Beyond the First Law, however, the exact details of these laws were not well understood.

First Law of Time
The First Law of Time specifically prohibited a Time Lord from meeting a former incarnation of themselves. (DW: The Three Doctors) Despite this, the Doctor on numerous occasions did just that — either accidentally (DW: Time Crash) or through Time Lord sanction. (DW: The Three Doctors, The Five Doctors) Charlotte Pollard broke the First Law of Time by traveling with the sixth version of the Doctor after having been the companion of the Doctor's eighth self, thus exposing the Doctor to his own future. (BFA: Brotherhood of the Daleks) Similarly, the Doctor may be said to have broken the First Law of Time by leaving messages for himself. (NA: No Future)

The First Law of Time was a moral as well as a legal one. (NA: Love and War)

Other Laws of Time
The Doctor once told Rose Tyler that "there used to be laws preventing this sort of thing" in reference to her interference with her own past. However, he failed to enumerate them. (DW: Father's Day) Likewise, the Brigadier's encounter with his past self was described by the Doctor's fifth self as being bad, but not as a specific violation of the First Law. (DW: Mawdryn Undead)
 * It was therefore possible that other laws of time were concerned with the Blinovitch Limitation Effect, the more generalized problem of any being meeting a past version of themselves. 


 * Likewise another law of time might have governed a single incarnation of a Time Lord from influencing an earlier version of the same incarnation, as the Doctor's ninth and tenth selves both willingly caused tiny loops in the timeline of those specific incarnations, without citing a violation of the First Law. Indeed, the Doctor once told Martha Jones that "crossing into established events is strictly forbidden, except for cheap tricks". (DW: Father's Day, Smith and Jones)

Another of the Laws of Time stated that an object from a non-existent timeline cannot be present in the current timeline. Cousin Justine of the Faction Paradox had a mask of such description. (EDA: Alien Bodies)

The Doctor's eighth self once said that learning "anything about future Gallifreyan history" would seriously unbalance the concept of causality. When he proceeded to nevertheless break this law, he claimed, "I'm breaking one of the major Laws of Time...It could be the third." (EDA: Alien Bodies)