Temporal grace

Temporal grace was the term used to describe the state in which the interior of TARDISes existed. Supposedly, it ensured no weapons could be used inside the Doctor's TARDIS. (TV: The Hand of Fear, TV: The Invasion of Time) It was linked to the TARDIS's telepathic circuits, and when fully functional, it would negate all hostile and aggressive actions within the ship. (PROSE: The Bodysnatchers)

Uses of temporal grace
Temporal grace stopped Eldrad from using her powers. (TV: The Hand of Fear)

Andred's staser did not work, although the Fourth Doctor attributed it to the relative dimensional stabiliser field. However, K9's laser and the Sontarans' guns did work. (TV: The Invasion of Time)

Temporal grace prevented Sha'ol from firing his disruptor. (PROSE: Mission: Impractical)

After the Seventh Doctor set off an explosion inside the SARDIT, he commented that the state of grace circuit still needed looking at. (PROSE: Cat's Cradle: Time's Crucible)

When nuclear missiles were launched on an alternate Earth, the Doctor materialised the TARDIS around the whole planet so that the temporal grace would prevent the missiles from exploding. (PROSE: Blood Heat)

Temporal grace, in conjunction with the TARDIS' medical nanites, freed Annie Carpenter from being possessed by the Scourge. (AUDIO: The Shadow of the Scourge)

Temporal grace prevented guns from being used against Nobody No-One, but since he was from another dimension, this did not apply to him. (AUDIO: The Word Lord)

The Eighth Doctor repaired the state of grace circuitry. When he drew several Skarasen into the TARDIS, he told his companions that the state of grace within the ship meant that the creatures couldn't harm them. (PROSE: The Bodysnatchers)

The Doctor later admitted to Sam that he had forgotten to reconnect the state of grace circuitry when they were faced with a fusion bomb the Daleks had planted. (PROSE: War of the Daleks)

In another universe, General Sontar tried to kill the Doctor by breaking his neck, but he survived due to the TARDIS' state of grace. (PROSE: The Infinity Doctors)

Failures of temporal grace
Despite the Doctor's claims, on many occasions it seemed that no such defence existed.

Sontarans (TV: The Invasion of Time), Cybermen (TV: Earthshock, Attack of the Cybermen), a Terileptil android (TV: The Visitation) and a Dalek (TV: The Parting of the Ways) discharged their guns in the TARDIS.

Nyssa later asked the Fifth Doctor about temporal grace not stopping the Cybermen from firing weapons inside the TARDIS, but he only replied, "Nobody's perfect." (TV: Arc of Infinity) Ace also questioned the Doctor about temporal grace after the interior nearly disintegrated. (PROSE: Blood Heat)

After Mels shot the TARDIS console, the Eleventh Doctor admitted that the presence of temporal grace in the TARDIS was (at least at that point) a "clever lie". (TV: Let's Kill Hitler)