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.