## Monday, March 14, 2011

### Time travel and interlibrary loan

The following argument is valid. I don't vouch for soundness!

1. (Premise) If I ought to A, then I can A.
2. (Premise) If I have received an interlibrary loan item, I ought to return it on or before its due date.
3. (Premise) If I can return an item on a date before today's date, then time travel is possible.
4. (Premise) Today I have received an interlibrary loan item due on March 8, 2011.
5. So, I ought to return an item on or before March 8, 2011.
6. So, I can return an item on or before March 8, 2011.
7. (Premise) Any date on or before March 8, 2011 is before today's date.
8. So, time travel is possible.

Premises (3), (4) and (7) appear to be true.

Leonhard said...

Well, I don't think your arguments proves that time travel is possible. It proves that, given the premises, you can return the loan items today OR before. It doesn't tell which of these is possible, merely that the conjunction is possible.

The sentence "I can return the books today or I can return them yesterday" conveys in the usual conversational sense that either possibility is true, but here in your argument it merely means that atleast one of them is possible.

In other words, whether or not time travel is possible, the sentence "I can return the books today or I can return the books yesterday" will be true as long as you can return the books today.

At least I think that's the problem. So premise 3 is wrong. You can return the books today (because you ought), or you can return them at any prior time (if time travelling is possible).

محب المعرفة said...

i cant under stand premise 3 ?
how is time travel possible regarding to this premise ?

Alexander R Pruss said...

Leonhard:

There was a slip up in one of the premises, which I've now fixed. :-)

محب المعرفة:

Well, I can't think of a way of returning the item prior to today without time travel. It doesn't have to be my time travel. It's enough if the item travels in time.

Leonhard said...

Slip up in my first comment i meant disjunction not conjunction. Your whole argument revolves around that you ought to "return the loan item today or earlier" and since "you can what you ought" then it possible for you to "return the item today or earlier". Since you can return it today then the sentence is true, and whether or not you return it prior to today depends on whether time traveling is possible. So premis 3 is flawed, it begs the question of whether time travel is possible.

It's like saying "I can travel through time or I can't - therefore I can travel through time"

Now if you could prove that if you couldn't return the book today that you still ought to, then it would follow that you could travel back through time. Unfortunately to prove this you'd need to assume that timetravelling is possible otherwise you oughten return the books but pay the fine. ;)

Alexander R Pruss said...

Since today is past the due date, it's too late to return it. :-)

Drew said...

I don't agree with premise 5. It should read: "I ought to have returned it before it was due"

If premise 1 is true, then I see no reason to believe premise 5 is tenselessly true unless I already believe the conclusion. In short, the argument is circular.

Alexander R Pruss said...

Drew:

But prior to receiving the item, I had no obligation to return it. :-)

Sarraclab said...

If ought implies can, this seems like an interesting way that ethical or deontological considerations could (in a way) precede metaphysical considerations. This seems like it could be relevant to the ontological argument. What about this: it seems plausible that knowing God is very good, and that whether or not God exists, it would be good to be able to know God (able in the sense of it being possible for me). So:

(1) I ought to be able to know God. (Premise)
(2) Ought implies can. (Premise)
(3) It is possible for me to be able to know God. (From 1,2)
(4) It is possibly possible for me to know God. (Reworking of 3)
(5) It is possible for me to know God. (From 4, and S4)
(6) It is possible that God exists. (From 5)
(7) Thus, God exists. (From 6, and S5)

Drew said...

Doesn't refute my point.

"Now" (from the perspective of this exercise) is March 11, 2011 so "I ought to have returned it before it was due" is a true statement

Alexander R Pruss said...

"Now" from the standpoint of the exercise is March 14, 2011. That's when I got the item due March 8, 2011. :-)

Drew said...

or we could go with:

5' So, I ought to return an item on or before March 8, 2011 if able.

That's the more accurate premise.

Alexander R Pruss said...

Or better, replace premise 2 with:

If I have received an interlibrary loan item, I ought to return it on or before its due date if I reasonably can.