Howling:TARDIS Explanation Please

Can someone explain to me why sometimes during the show, Rose will ask the Doctor if they can use the TARDIS to jump back a few hours before bad situation X started happening, and the Doctor will say they can't because they are part of the events now? What exactly does that mean? Why does that stop them from using the TARDIS? Is this something covered in the older seasons?


 * It's got to do with crossing your own timeline, and changing it. It's sometimes termed the Blinovitch Limitation Effect. DW: Day of the Daleks covers it...somewhat.
 * Alternatively they can't get back in the TARDIS and change things as this might lead to a Temporal paradox (see that article for more information and example).
 * The basic idea however, is the reason they can't jump back before events started going pear shaped is because they didn't see themselves doing so. If they went back in time then they would be changing their own past during situation X. --Tangerineduel 08:10, September 9, 2009 (UTC)