In the first part of Aztecs (which is in series one and we watched as a class) has many different elements throughout the episode, I will be focusing on the very tricky subject of changing the past. To understand the ideas behind changing the past there we have to first understand some of the concepts of time travel.
There are a couple of different ways to look at time travel; the first way is to understand time much like a time line, where one thing happens before the next before the next and since the thing in the present (and technically the future) going into the past will not change anything at all because what has yet to occur has already happened. Another way is to view time itself as a malleable object that can be bent and warped to the travelers will that allows them to go from one point to another. It is important to point out that this view allows for the changing of the past which means that the present and the future can be altered as well. Finally the last prominent time travel theory is the belief that when individuals travel forward in time they travel into the future of what their present currently allows, but when traveling into the past something very different happens. Based off of this theory people who travel backwards in time automatically change the past creating an alternate present/future while still allowing the present/future that existed/would occur to exist and run its course. This is quite possibly one of the most confusing theories because it allows for every single decision in the present, past and future to create a new and different reality that would have not existed depending on the outcome of the decision.
In the first part of the Aztecs The Doctor says that time can not be changed because it has already molded the present and has already occurred which models the first theory that I outlined for the most part. I find this to be a very difficult pill to swallow because if that is the case then there is no other point of traveling from one point in time to another other than educational purposes. It also means that it would rob time travelers of their free will because it would vastly limit the amount of interaction the travelers could have in the past or future. It also would mean that people in the present are unable to change what we perceive as the future because every present is the past of some time. With every present being the past of some time then every decision we make has already been made and how we feel about things does not matter, this also means that not only do we have no control over our lives but that we have no true responsibility for our actions.