Okay, so so far we've covered Lewis' account of what time travel actually involves. And we've covered Lewis' attempt at trying to diffuse the appeal of grandfather paradoxes. So so far the account is this. Traveling to the past is logically possible provided that what the traveler does in the past is consistent with the history whence the traveler comes. Now at this point, you might be thinking, well hang on. If the travelers actions already exist in the past, and some say before the traveler sets out. Or at least if travelers actions exist at external times, early in the travelers departure, how can the traveler possibly have any impact on the past? People sometimes hear Lewis' analysis and think well surely the traveler is just a sort of pre-preprogrammed robot, completely predetermined to go through a completely rigid set of actions. Well, still people might think that a traveler in the past is doomed to be a sort of ghost, forced to witness events the powerless to intervene. But Lewis thinks that it’s possible for a traveler to be really concretely present in the past. A properly functioning human agent with intentions and wishes and choices and to make a difference to the past. You have to be careful how we imagine the impact of a traveler in the past. Lewis distinguishes between two senses in which it could be said a traveler could change the past. What I'm going to call replacement change and counterfactual change. In a nutshell, Lewis says that a traveler in the past cannot effect replacement changes but can effect counterfactual changes. What's a replacement change? Consider a perfectly ordinary object, like a glass. If I were to drop a glass from waist height onto a concrete floor, and the glass shatters, I would have replaced an intact glass with a set of glass fragments. I would have effected a replacement change in events. There was an intact glass, the intact glass is shattered. The intact glass has gone away, and in its stead it's being replaced by a set of glass fragments. Now Lewis thinks that replacement changes can happen to concrete objects, but not to times. You can't replacement change any time, past, present or future. Suppose you make a plan to meet a friend for lunch at twelve o'clock at a certain restaurant. And then you get a text from your friend to say I'm sorry, I can't make lunch today, can we meet tomorrow? Well that hasn't replacement changed the future. It's not that you did in the future meet at a certain restaurant and then that future somehow went away. So Lewis says yes you can't affect replacement changes in the past but you can't affect replacement changes in the future either. Replacement changes can only happen to complete objects. You can replace a concrete object, like an intact glass with a set of glass fragments. But complete objects are not the same as times. So that's replacement change. Counterfactual change may be a little bit harder to get a handle on. But counterfactual change is the impact that you have assessed in terms of what would have happened, counterfactually, if you hadn't been present. One of the things that enabled me to be on time for this session this morning was that my alarm clock went off on time. But if the alarm clock hadn't gone off, I would've been late. So I can assert the counterfactual if my alarm clock had broken I wouldn't have been on time. So when my alarm went off clearly had an impact on my ability to attend this session on time. If my alarm clock had broken I would have been late. So, in a sense, the alarm has changed the course of my day. But that change is not to be assessed in replacement terms. It's not that there was an original version of events where my alarm clock didn't go off and I was late. And then somehow my alarm clock did go off and history was replacement changed, and I was on time. Rather the impact the alarm clock had can be assessed counterfactually. This morning really happened. What we see happened is my alarm clock going off on time. But if it had happened differently, history would have unfolded differently. Another example. Historians who treated the period, not least Arthur Wellesley, the Duke of Wellington. Maintain that a crucial fact in determining the outcome of the Battle of Waterloo was the arrival in the late afternoon at the Battle of Prussian forces under the command of Field Marshal Blucher. Wellington himself frequently said that if Blucher had been late, Napoleon would have won. So that's clearly a counterfactual conditional. If Blucher had been late, Napoleon would have won. Again, it's not that Blucher made a replacement change to the Battle of Waterloo. It's not that Waterloo originally issued in a French victory and then Blue Coats forces arrived and the French victory somehow was made such that it never was and an allied victory took it's place. Waterloo happened only once, with a victory for the allies. But an important factor in that victory was the arrival of Marshal Blucher. So we can see that Blucher's arrival changed the course of history. But it changed it in the counterfactual sense, not in the replacement sense. Okay, we've got two senses of change, replacement change, counterfactual change. Lewis maintains that time travelers can have an impact on the past in the counterfactual sense. The presence of a traveler might make history different from what it would have been if the traveler hadn't been there. Going back to my attempt at assassinating Hitler. Suppose my time machine deposits me in Vienna at 1908 with a great flash of light. And I've arrived so close to Hitler that Hitler sees a flash of light and recoils. He steps back in shock out of the path of a tram that would otherwise have cut him down. In this case, I can assert the counterfactual if I hadn't traveled back in time, Hitler would've died. So in this case, I clearly had a counterfactual impact on history. I have, albeit unwittingly, been partly responsible for Hitler's survival. So it could well be that the course of history could consistently contain the counterfactual impact of the presence of time travelers. Another example, suppose that I travel back to 1864 and I bump into Lincoln. Lincoln's about to give the mutuled famous words at the Gettysburg address, but he's unsure of which version to give. He has a choice between the famous version that history recalls and another version. And I say to him, go with the version that's starts these great resident sentences about how this nation was conceived in liberty that will go down very well. Lincoln takes my advice, and the other version of the speech is big. But suppose if I hadn't intervened, Lincoln would have recited a different version of the Gettysburg address. Well I have clearly had an impact on history. History is different as a result of my effort. But I've not replaced anything. I've not made one version of the Gettysburg Address disappear and another version take its place. The Gettysburg Address happens once and once only. But I've still changed the course of history.
