Originally posted by Mrja84
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The tenth Doctor didn't remember until the situation arose. By this point the timelines between him and his past self were in sync and the tenth Doctor could remember.
The same thing occurs with other cross Doctor stories. The simple reasons behind this is as a protection mechanism for a timelord's own personal time line. If The Doctor could remember all the events where he crossed paths with himself then he could interfere with them and change them in such a way to create a paradox.
This isn't even necessarily something that occurs naturally as a part of temporal science, but as a in built feature in a Timelord physiology like regeneration and a natural ability to have a sixth sense in regards to time.
The general rule of thumb therefore is: A past Doctor will not remember his future self. If two regenerations meet more than once than memories of the original meeting or meetings will resurface as his own timelines sync up.
The eldest version present will retain all memories of the incident but will only gain access to the memories as the event unfolds, starting as a vague feeling of what is going to happen and eventually becoming the full memory of what will happen, like knowing how to avoid the TARDIS explosion, or when to jump through the time vortex to meet his former self.
He may know what to do, but not necessarily what will happen next as a consequence. Instead it's as if he runs on instinct.
Once the event has occurred, all past incarnations will lose their memories once they are outside the presence of eachother and the timelines lose sync.
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