DO CRY OVER SPILT MILK: POSSIBLY YOU CAN CHANGE THE PAST Peter B. M. Vranas Iowa State University Central APA, 28 April 2005

INTRODUCTION It is widely agreed that changing the past is impossible. Even if time travel or backward causation is possible: it is then possible to affect but not to change the past. I will: Ê Distinguish transforming from replacing (and both from affecting) the past. Ë Argue that replacing the past counts (interestingly) as changing it. Ì Argue that, if affecting the past is possible, then so is replacing the past.

OVERVIEW Part 1 AFFECTING, TRANSFORMING, AND REPLACING THE PAST Part 2 REPLACING THE PAST COUNTS AS CHANGING IT Part 3 IF AFFECTING THE PAST IS POSSIBLE, THEN SO IS REPLACING IT

AFFECTING VS CHANGING THE PAST l I affect the past iff I have a causal effect on the past (and thus only if backward causation occurs). l I change the past iff I perform now an action which makes the past (qualitatively) different from what it is. l So I never in fact change the past, but this does not settle the questions of (1) whether it is possible that I change the past, and (2) whether I can change the past.

TRANSFORMING VS REPLACING THE PAST l It is possible that I change the past iff there is a possible world w in which I make the past (in that world) different from what it is. But different from what it is in w or in a? Ê It is possible that I transform the past iff  w  t(at t I do something which makes the past of t in w different from the past of t in w). Ë It is possible that I replace the past iff  w  t(at t I do something which makes the past of t in w different from the past of t in a).

OVERVIEW Part 1 AFFECTING, TRANSFORMING, AND REPLACING THE PAST Part 2 REPLACING THE PAST COUNTS AS CHANGING IT Part 3 IF AFFECTING THE PAST IS POSSIBLE, THEN SO IS REPLACING IT

ANALOGY WITH REPLACING THE FUTURE l Suppose I’ll die in If I killed myself to- morrow, I would change the time of my death, so I would change the future in the sense of replacing: bringing about a nonactual future. l Changing the future is more interestingly un- derstood as replacing than as transforming it. l Similarly, if I went back to 1965 and changed the time of my birth, I would change the past in the sense of replacing: bringing about a nonactual past.

OBJECTION: THEORIES OF CHANGE l 3-D theory (Mellor): A thing changes only if it has different properties at different times. l 4-D theory (Lewis): A thing changes only if there is a qualitative difference between different temporal parts of it. l The objection: Replacing the past does not amount to making it the case that the past has different properties at different times or that there is a qualitative difference between different temporal parts of the past.

REPLY TO THE OBJECTION l My reply: When a new government takes of- fice, the government changes: the old govern- ment is replaced with (not transformed into) a new one. So replacing counts as changing, despite standard theories of change. l A response: Replacing a government differs from replacing the past. l My reply: The analogy need not show that replacing the past counts as changing, but does refute the objection from theories of change.

OVERVIEW Part 1 AFFECTING, TRANSFORMING, AND REPLACING THE PAST Part 2 REPLACING THE PAST COUNTS AS CHANGING IT Part 3 IF AFFECTING THE PAST IS POSSIBLE, THEN SO IS REPLACING IT

IF AFFECTING THE PAST IS POSSIBLE, SO IS REPLACING IT Premise: If it is possible to have a causal effect e on the past, then is possible to have an incompatible causal effect e' on the past. Conclusion: If it is possible to affect the past, then it is possible to replace the past. Proof: Suppose  w(at t I cause e in the past of t in w). Then  w'(at t I cause e', incompatible with e, in the past of t in w'). Suppose e does not obtain in a. Then  w(at t I cause the past of t in w—which contains e—to differ from the past of t in a—which lacks e).

OBJECTION: THE PAST IS ALREADY ACTUALIZED l The objection: Necessary, the past is already actualized, so it is impossible to now replace the past, to actualize a nonactual past. l My reply: (1) If an event is actualized when it occurs, to claim that in w I make the w-past differ from the a-past is not to deny that in w the w-past, and in a the a-past, is already actualized. (2) If an event is actualized when it is caused, then the past need not be fully actualized if backward causation is possible.

IF I CAN AFFECT THE PAST, THEN I CAN REPLACE IT l I can do A  It is possible that I do A. Premise: I can have incompatible causal effects e and e' on the past. Conclusion: I can replace the past. Proof: Suppose e does not obtain in a. Since I can cause e, I can make the past differ from the actual past: I can replace the past. l Objection: I can cause only what has already occurred. Reply: The objection presupposes that it is impossible to replace the past.

CONCLUSION: THE FUTILITY OBJECTION l The objection: Suppose I now can prevent my father’s death in What is the point of telling me this? I know I will not prevent it. l My reply: Suppose I now can prevent my being fired in What is the point of telling me this if I know I will not prevent it? l The point may be to let me know that it will be my own fault: I will not exercise my power to prevent it. No futility need be involved. Similarly about the past.

WHY I NEVER IN FACT REPLACE THE PAST l It is possible that I replace the past iff  w  t(at t I make the past of t in w differ from the past of t in a). l I in fact replace the past iff (in a)  t(at t I make the past of t in a differ from the past of t in a). l It is possible that I transform the past iff  w  t(at t I make the past of t in w differ from the past of t in w). l I in fact transform the past iff (in a)  t(at t I make the past of t in a differ from the past of t in a).