Phil 105: Science and Philosophy - Fall 2004 Lectures

!You will be sorely disappointed if you think that these are a substitute for attending class!

Topic Four: Time

1. Logical consistency

• Consider an inconsistent time travel story – the story is impossible, but what in particular is impossible about it?
• Any individual step?
• The portal?
• The steps as a whole?

• What does physics have to do with what I can do?
• Physics: laws determine what is possible in the future, given the physical state now – examples?
  
• Whatever properties I have are determined by the physical state now (especially mine) – examples?
  
• Well, perhaps our mental/spiritual state isn't so determined, but I will suppose that the physical state plus the mental/spiritual state determines what is possible in future.

• So,
  
• given the laws,
  
• given travel through a portal,
  
• it must be that any earlier states that would produce an inconsistency are impossible – the only earlier states that are possible are those in which I fail to stop myself, somehow or other.

• Does that make sense?
• Yes, in the block.
  
• Yes if the block is wrong – just conditioning the past on the future.

2. In what way am I unable to stop earlier myself?

• What we've seen is that because of the earlier state I am in some sense unable to stop my earlier self – but that seems wrong! I might actually fail but surely in principle, in the ordinary way of speaking, I am able to!

• Consider how our ordinary way of ascribing abilities meshes with our physical state:
• Many 'fine grained' physical states …
  
• … correspond to any one 'coarse grained' set of abilities.
  
• That is, there are many different ways I can be physically constructed such that we'd still say 'I can do x', because we are always ignorant of the exact physical state.
• And hence while we can truly say 'she can do y', she still might fail – her physical state, and the state of the world, and the laws, might in fact make her fail.

• And that is the solution to our puzzle:
  
• I am able to stop myself in the ordinary sense – I could stop someone with the same abilities (a corase grained duplicate),
  
• while I am not able to in the physics sense – given my physical state and the physical state of my earlier self and the laws, I will certainly fail.