First Law of Time

The First Law of Time, (TV: The Three Doctors) also called the Protocols of Linearity, was the most important and widely-discussed of the Protocols of the Great Houses, which were hard-wired into the Spiral Politic during the anchoring of the thread. (PROSE: The Book of the War) It forbade Gallifrey's present from interacting with its own subjective past or future. (PROSE: Cat's Cradle: Time's Crucible, Lungbarrow, Alien Bodies, The Book of the War)

One consequence was that Time Lords could not meet each other out of temporal sequence (PROSE: Goth Opera) or meet their former selves. (TV: The Three Doctors) That restriction could even apply to non-Time-Lords: the Fourth Doctor refused to return Eldrad to her native time because that would have been a "distortion of history" that "contravened the First Law of Time". (TV: The Hand of Fear) The Ninth Doctor once told Rose Tyler that there "used to be" laws banning interference with one's own past. (TV: Father's Day) Likewise, the Fifth Doctor described the Brigadier's encounter with his past self as being bad. (TV: Mawdryn Undead) The Eleventh Doctor said that a time traveller's own grave was the one place they could never go. (TV: The Name of the Doctor)

Another consequence of the Protocols of Linearity was that if a Homeworlder were to leave the Homeworld for five years, though they could theoretically return moments after they left, upon their return they would inevitably find that five years had passed there as well. In effect, whenever an agent entered an area of time outside the Homeworld, their relative histories would be temporarily linked so that their "present"s would be indistinguishable, despite being aeons apart. This rule applied to other time-active powers as well, including Faction Paradox and the enemy, which many speculated could be due to the Houses' inability to revoke the Protocols, an agreement between the parties, or mutual fear of the consequences of non-linearity. (PROSE: The Book of the War)

Violations
This law could be bent without breaking: on several occasions, objects from the Homeworld's past were "hooked" and dragged into the present. (PROSE: The Book of the War) For instance, the Doctor's incarnations were pulled together several times through Time Lord sanction (TV: The Three Doctors, The Five Doctors, The Two Doctors, PROSE: World Game) or by accident. (TV: Time Crash, Twice Upon a Time)

The Eighth Doctor chastised Sebastian Grayle for breaking the First Law of Time after Grayle told him they would meet in the future. (AUDIO: Seasons of Fear) The Eighth Doctor's companion Charlotte Pollard similarly broke the First Law by later travelling with his sixth incarnation, thus exposing the Doctor to his own future. (AUDIO: Brotherhood of the Daleks)

In the years before the War in Heaven, the Great Houses encountered their future counterparts several times. For instance, Thessalia interacted with War-era agents of Faction Paradox during a Violent Unknown Event on Zo la Domini (PROSE: The Book of the War) The Eighth Doctor encountered the War several times while it was in his future, (PROSE: Alien Bodies, Unnatural History, The Taking of Planet 5) saying that he was "breaking one of the major Laws of Time... It could be the third." (PROSE: Alien Bodies)

After Faction Paradox signed the Gregorian Compact with George II in 1752, agents of the Faction and the Great Houses alike found it difficult to penetrate Earth's causality for the next seventy years: observing this period would be too much like looking into their own future. However, Cousin Belial slipped through the lock on the late 1700s by being reborn in 1782. (PROSE: The Book of the War)

Robert Scarratt was known to test the limits of linearity by using timeships to go AWOL for periods totaling at least a year of subjective time. (PROSE: The Book of the War)

When the Remote left Faction Paradox, they became divorced from linearity. As a result, Compassion could be born during the War but go on to travel with an old-fashioned timeship in the pre-War era. (PROSE: Interference, The Book of the War)

During his ninth and tenth incarnations, the Doctor willingly caused tiny loops in his own timeline. (TV: Father's Day, Smith and Jones) In contrast, the Eleventh Doctor later told Carter that he'd "best not" cross his own timestream; (TV: The Wedding of River Song) the TARDIS later tried to stop him from visiting his grave on Trenzalore. He believed this visit had made his fate unchangeable, (TV: The Name of the Doctor) though with the Time Lords' help he was able to change the future. (TV: The Time of the Doctor)

The Eleventh Doctor was content to bring a young Kazran Sardick into his own future in an attempt to change his past. (TV A Christmas Carol)

Due to the actions of the Moment, the Doctor crossed his own timestream in a big way when he met the War Doctor and the Tenth Doctor and they shared an adventure together. They later broke the law in an even bigger way by calling forth thirteen incarnations of themselves to help with their plan. (TV: The Day of the Doctor)

The Twelfth Doctor refused to help Clara Oswald save Danny Pink as it meant crossing her own timeline which was a bad idea. (TV: Dark Water) Unknown to the Doctor, he had earlier accidentally crossed his own timeline when Clara piloted the TARDIS from the end of the universe and they landed in a barn where the young First Doctor was crying. However, Clara kept the Doctor from knowing the truth and convinced him never to return to find out when and where they had travelled to. (TV: Listen)

Other references
The Seventh Doctor stated that interfering in Gallifrey's past time travel experiments was against the First Law; (PROSE: Cat's Cradle: Time's Crucible) by several accounts, he was present during those experiments. (PROSE: Remembrance of the Daleks, Lungbarrow, COMIC: The Lost Dimension)

The Fourth Doctor said it was a criminal act on Gallifrey to meddle with one's own time coordinates. (PROSE: Master of the Blackhole)

The Seventh Doctor once joked that the first law of space-time travel was "avoid voids". (PROSE: The Highest Science)