Wednesday, April 20, 2011

Special Relativity and Perdurantism

There seems to be a problem for the conjunction of Special Relativity and perdurantism. Maybe this is a standard problem that has a standard solution?

Let's say that being bent is an intrinsic property. Perdurantists of the sort I am interested in think that Socrates is bent at a time in virtue of an instantaneous temporal part of him being bent (I think the argument can be made to work with thin but not instantaneous parts, but it's a little more complicated). Therefore:
  1. x is bent at t only if the temporal part of x at t is bent simpliciter.
The following also seems like something perdurantists should say:
  1. x is bent simpliciter only if every temporal part of x is bent simpliciter.
Now, we need to add some premises about the interaction of Special Relativity and time.
  1. There is a one-to-one correspondence between times and maximal spacelike hypersurfaces such that one exists at a time if and only if one at least partly occupies the corresponding hypersurface.
Given a time t, let H(t) be the corresponding maximal spacelike hypersurface. And if h is a maximal spacelike hypersurface, then let T(h) be the corresponding time. Write P(x,t) for the temporal part of x at t. Then:
  1. P(x,t) is wholly contained within H(t) and if z is a spacetime point in H(t) and within x, then z is within P(x,t)
and, plausibly:
  1. If a point within x is within a maximal spacelike hypersurface h, then P(x,T(h)) exists.
Now suppose we have Special Relativity, so we're in a Minkowski spacetime. Then:
  1. For any point z in spacetime, there are three maximal spacelike hypersurfaces h1, h2 and h3 whose intersection contains no points other than z.
Add this obvious premise:
  1. No object wholly contained within a single spacetime point is bent simpliciter.
Finally, for a reductio, suppose:
  1. x is an object that is bent at t.
Choose a point z within P(x,t) and choose three spacelike hypersurfaces h1, h2 and h3 whose intersection contains z and only z (by 6). Now define the following sequence of objects, which exist by 4 and 5:
  • x1=P(x,t)
  • x2=P(x1,T(h1))
  • x3=P(x2,T(h2))
  • x4=P(x3,T(h3))
Observe that x4 is wholly contained in the intersection of the three hypersurfaces h1, h2 and h3, and hence:
  1. x4 is wholly at z.
  2. It is not the case that x4 is bent simpliciter.
  1. x1 is bent simpliciter. (By 1 and 8)
  2. x2 is bent simpliciter. (By 2 and 11)
  3. x3 is bent simpliciter. (By 2 and 12)
  4. x4 is bent simpliciter. (By 2 and 13)
    Since 14 contradicts 10, we have a problem. It seems the perdurantist cannot have any objects that are bent at any time in a Minkowski spacetime. This is a problem for the perdurantist.

    If I were a perdurantist, I'd deny 2, and maintain that an object can be bent simpliciter despite having temporal parts that are bent and temporal parts that are not bent. But I would not be comfortable with maintaining this. I would take this to increase the cost of perdurantism.

    What is ironic here is that it is often thought that endurantism is what has trouble with Relativity.

    Friday, April 15, 2011

    One-Category Abundant Platonism

    Standard Abundant Platonism (SAP) holds that to every predicate there corresponds a property, and items satisfy the predicate if and only if they exemplify the property.  Moreover, it holds that exemplifiers are not explanatorily prior to what they exemplify.  Normally, we think of SAP as a two-category theory: individuals and properties.

    But here is a suspicion I have.  Little if any explanatory work is being done by the distinction between individuals and properties.  The serious explanatory work is all being done by the relation of exemplification.  Here are two examples.

    1. Standard Platonists say that x and y are exactly alike in some respect if and only if there is some property P such that x exemplifies P and y exemplifies P.  But drop the word "property" from the previous sentence, and we have an account of exact alikeness that is even better: x and y are exactly alike in some respect if and only if there is a z such that x exemplifies z and y exemplifies z.  This is extensionally just as good, but simpler. (One can do more complex stuff about determinates and determinables to get resemblance in some specific respect, but again that doesn't need the concept of property, just the relation of being a determinable of.)

    2. Standard Platonists say that to each predicate F there corresponds a property Fness, and that x is F if and only if, and if so because, x exemplifies Fness (we should probably have an exception to the "because" clause when Fness is exemplification).  But change "there corresponds a property Fness" to "there corresponds an entity Fness", and this works just as well as an account of predication.

    Besides, the concepts of "individual" and "property" are foggy.  (We might try to say: "x is an individual if and only if x cannot be exemplified."  But that doesn't work for abundant Platonism, as abundant Platonism will have properties like being a square circle.)

    So, if you're going to be a Platonist, why be a two-category abundant Platonist?  Why not be a one-category abundant Platonist instead?