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 appeared grandfather paradoxes. So, so far, the account is this: travel into 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 traveler's actions already exist in the past, and some say it's before the travelers set sight, or at least if the traveler's actions exist at external times earlier in the traveler's departure, how could the traveler have any impact on the past. People sometimes hear Lewis' analysis and think, well surely the traveler is just a sort of pre-programmed robot, completely pre-determined to go through a completely rigid set of actions. Or what still, people might think that a traveler in the past, is doomed to be some sort of ghost, forced to witness events, but be powerless to intervene. Well Lewis thinks that it's possible for a traveler be really concretely present in the past, a proper functioning human agent with intentions, and wishes, and choices, and to make a difference to the past. We 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 that a traveler could change the past, what I'm going to call replacement change and counterfactual change. In a nut shell, Lewis says that a traveler in the past cannot effect replacement changes, but can effect counterfactual changes. Well 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 affected 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 And in its stead, it's been replaced by, a set of glass fragments.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 12 o' clock at a certain restaurant. And then, you get a text from your friend saying, 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 the replacement changes in the past, but you can't affect the replacement changes in the future either. Replacement changes can only happen to concrete objects. You can replace a concrete object, like an intact glass, with a set of glass fragments, but concrete objects are not the same as times. So that's replacement change. Counterfactual change, maybe 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 have 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, or 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 only happened once, and it happened with 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, maintained there's a crucial factor 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 Marshall 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 Blucher's forces arrived and the French victory somehow was made such that it never was and an Allied victory took its place. Waterloo happened only once, with a victory for the Allies. But an important factor in that victory, was the arrival of Marshall 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, placement 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 in 1908 with a great flash of light. And I've arrived so close to Hitler that Hist-, 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 could assert the counterfactual, if I hadn't traveled back in time, Hitler would have 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 kinds of factual 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 world famous words, that of the Gettysburg Address, but he's unsure of which version to give. He has a choice between the famous version history recalls and another version. And I say to him, Abe, go with the version that starts, these great resonant sentences, about how this nation was conceived in liberty. That will go down very well. And Lincoln takes my advice. And the other version, of the speech is binned. But suppose if I hadn't intervened, Lincoln would have recited a different version of the Gettysburg Address. Well, I could have had an impact on history, history is different as a result of my efforts. 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.
