Laws of Time

The Laws of Time were a set of guidelines and laws guiding what a time traveller may do. As opposed to physical Laws, which were observations of constants of behavior found in nature (ie The Laws of Motion or Gravitation), the Laws of Time were rules or guidelines 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 is unknown how those law were enforced, or what the exact punishment for breaking them was. It is possible the effects of breaking them were serious enough that they would themselves serve as a deterrent.

The Doctor has 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 include 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 has intentionally interfered with a fixed point, under the rationale that, being the last surviving Time Lord, the Laws of Time were now his to command. Ultimately, he is unable to prevent Brooke from committing suicide, thereby allowing the timeline to unfold, with only a few minor changes. (DW: The Waters of Mars) However, the 2009 Dalek Invasion of Earth, despite the fact it led to Brooke becoming a fixed point in time, was not, itself, a fixed point and therefore the Doctor was able to stop it from continuing. (DW: Journey's End).

The resolution of the Last Great Time War, which involves the destruction of Gallifrey, has been "time locked", rendering it technically an unalterable -- and inaccessible -- point in space and time. Nonetheless, Dalek Caan was able to circumvent the restriction and retrieve Davros, though at the cost of his sanity. (DW: The Stolen Earth/Journey's End)

The Time Lords themselves once sanctioned what could be the greatest-ever attempt at interfering with a fixed point in time, when they assigned the Doctor the task of preventing the creation of the Daleks. The Laws of Time, however, remained intact as the Doctor did not do so. (DW: Genesis of the Daleks)

Exactly what constitutes a fixed point in time is 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

One of the Laws of Time supposedly prevented Time Lords from meeting their previous selves, but this was broken several times in emergencies (DW: The Three Doctors, The Five Doctors, BFA: The Sirens of Time, IDW: Room with a Deja View) and at least twice by accident (DW: The Two Doctors and Time Crash). Notably, when the Doctor's Fifth and Tenth incarnations met following the destruction of Gallifrey (DW: Time Crash) it threatened to cause damage to the space-time continuum, suggesting a possible rationale for the law (implying the Time Lords likely kept those effects in check during "sanctioned" incidents).

One 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)

Learning "anything about future Gallifreyan history, causality could be damaged beyond conception...I'm breaking one of the major Laws of Time...It could be the third." -The Doctor (EDA: Alien Bodies)

First Law of Time

The First Law of Time prohibits a Time Lord from interfering with his own timestream by, among other things, interacting with past incarnations. Despite this, the Doctor has on numerous occasions done just that, either accidentally (DW: Time Crash) or through Time Lord sanction. (DW: The Three Doctors, The Five Doctors) Rose Tyler broke the first law by preventing her father's death (DW: Father's Day), which had negative consequences. Brigadier Alistair Gordon Lethbridge-Stewart once encountered himself from a different timestream, resulting in his younger self suffering amnesia. (DW: Mawdryn Undead)

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

• The Doctor may be said to have broken the First Law of Time by leaving messages for himself. (NA: No Future)