Philosophy Asked by AnduinWilde on March 5, 2021

In Quantified modal logic,

“constancy’s defenders can point to certain powerful arguments in its favor.

Here’s a quick sketch of one such argument. First, the following seems to be a logical truth:

Ted = Ted

But it follows from this that:

∃y y = Ted

This latter formula, too, is therefore a logical truth. But if φ is a logical truth

then so is □φ (recall the rule of necessitation from chapter 6). So we may infer that the following is a logical truth:

□∃y y = Ted

Now, nothing in this argument depended on any special features of me. We may therefore conclude that the reasoning holds good for every object; and

so ∀x□∃y y = x is indeed a logical truth. Since, therefore, every object exists necessarily, it should come as no surprise that there are things that might have

been ghosts, dragons, and so on— for if there had been a ghost, it would have necessarily existed, and thus must actually exist.

This and other related arguements have apparently wild conclusions, but they cannot be lightly dismissed,for it is hard to say exactly where they go wrong (if they go wrong at all!).8

8 On this topic see Prior (1967, 149-151); Plantinga (1983); Fine (1985); Linsky and Zalta (1994,1996);Williamson (1998, 2002).”

——Sider, Logic for Philosophers, Oxford, 2010, p307

- Would this count as a valid reasoning?

(1) For all x, x=x.

(2) the whole number between 4 and 5 = the whole number between 4 and 5.

(3) So there is some y such that y is the whole number between 4 and 5.

But in which way does it differ from Sider's reasoning?

I think that no name ( or description) should be substituted for

*x*in sentence (1) unless this name actually refers to something.

In that case, the conclusion would be trivial in case of actually existent objects, and could not be arrived at in case there is no object the " name " refers to.

Answered by user37859 on March 5, 2021

The magician asks for a volunteer to offer their watch. He then takes the watch and puts it in a black cloth bag. He then smashes the smithereens out of the black bag, using a large metal hammer. He then opens the bag. You know that watch=watch, so you expect that there exists a watch in the black cloth bag.

But the magician is not a capable magician. He made a mistake.

So he hands you back a pile of formerly-parts-of-a-watch debris. The watch is no more. You sadly recall your trip to the watch factory, where you saw this very watch constructed. Never more will you know what time your favorite philosophy related TV show starts.

So it clearly is not true without a time component. You may then infer that, since there is one category of "it's not true" cases, there could well be more.

You can still say watch=watch and have it be true, even when the watch is not existent. In this case, the nature of the watch is "does not exist."

Answered by puppetsock on March 5, 2021

You have two logical formulas here. Note that Ted, = and y are just symbols, they don't, in the logic, correspond to any actual objects. What we need is a model for that.

I am now proposing this model:

=(A,B) is always false The set of object in my universe is empty.

Under this model your formulas are both false.

Let's now take = to be what you expect, the identity relation. Still in a universe with no objects. The first relation is true but the second false.

Finally, a universe where there is some person called Ted that is equal with himself. Of course in that universe Ted exists.

=> Don't mix up syntax and semantics, you need both. You get strange answers if you forget that first-order logic makes no assumptions wrt. the model.

You can clearly see something is wrong when you come to the conclusion unicorns exist in our world, when in fact they don't. They exist in a model world if you say they do. But for the logic, there is no distinction between whether an object is a number, a unicorn, a grain of sand, or russels teapot.

Answered by kutschkem on March 5, 2021

I don't think everything exists. It only exists if it can exist. But since dragons never existed just before we imagined them doesn't mean they can or ever will exist.

Answered by kikds on March 5, 2021

If I'm understanding correctly, I could use your logic to make the argument that unicorns exist as follows:

- unicorn = unicorn
- there exists some x = unicorn

therefore, unicorns exist

I agree that (1) holds. I also agree that (2) implies the conclusion. Step (2) strikes me as more problematic. Why is there some x = unicorn? If I assume that you are using (1), presumably it's because you can choose unicorn = x. But then you are assuming your conclusion and your reasoning is circular.

Answered by Stack User on March 5, 2021

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