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Consider the statement "I'm moving". It seems to me that this statement can be both true and false. That is, because motion is relative, I may not be moving relative to the Earth (i.e. standing still), but I'm always moving relative to the Sun, for example (or virtually any heavenly body for that matter). It looks as though the whole thing is a kind of a cheap trick, but what is it exactly?

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It is exactly not cheap and not trick. It is major property of the language. Infinite (almost) flexibility of interpretation. Cheapness was in your desire to EVALUATE if its true or not. In sentence itself was nothing wrong. So to IMPROVE price we need to advance in the way we EVALUATE this sentence. Last but not least you assume that if something is correct it can not be part of something wrong. and this is not true. –  Asphir Dom Apr 25 at 14:20
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Before getting angry and things, read the question carefully and understand that I didn't say anything contrary to what you're saying right now. –  user132181 Apr 25 at 14:49
    
FYI i did not get angry :) SO. What is your question about? If you understood everything right i don't see the matter in question. Where is puzzle? –  Asphir Dom Apr 25 at 14:55
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You got an emotional reaction out of Asphir, so he found you to be moving. I on the other hand am a dispassionate robot, so I didn't find you to be moving at all. On the other hand, Gone With the Wind is moving even for a dispassionate robot. –  MatthewMartin Apr 25 at 15:54
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Given the context is not included, as per the accepted answer, I would say it's a case of lying by omission - the 'trick' being used in this instance. –  Zibbobz Apr 25 at 17:01
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3 Answers 3

up vote 10 down vote accepted

1 Let's start with a simpler case first. While it is true that the statement

(a) "I am moving"

is true at some points in time and false at others, it doesn't mean that given any point t in time we have:

(b) "I am moving at time t " ∧ "I am not moving at time t ".

Given any time t, either you are moving at time t or you are not. The fact that there are times at which you are moving and that there are times at which you are not moving, does not contradict that fact.

2 Similarly, "I am moving" can be true with respect to some objects in space and false with respect to others. For example, if you're on a train, you're moving with respect to the external environment of the train, but are stationary with respect to the person sitting next to you. It doesn't follow from this that:

(c) "I am moving with respect to an object o " ∧ "I am not moving with respect to that o ".

Given any object o in space, either you are moving with respect to o or you are not. The fact that there are objects (e.g. the Sun) with respect to which you are moving and that there are objects (e.g. the Earth) with respect to which you are not, does not contradict that fact.

3 The impression that you have that there is some trick being played here might be caused by the fact that the context of utterance is often implicit. The context determines things such as the time and place of the utterance, the person uttering the sentence, and so on. Once the context is made explicit, it's possible to make formally precise the idea that the same sentence can have different truth-values depending on different context-sensitive factors, such as the time of utterance and so on.

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So, "I'm moving" doesn't have a truth-value, after all? –  user132181 Apr 25 at 10:56
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It does, but it's truth-value is a function of its context. Is the sentence "p" true or false? It depends on the truth-assignment to 'p'; with respect to the environment {p := true}, 'p' is true, while with respect to the environment {p := false}, 'p' is false. It doesn't follow from these observations that 'p' has no truth-value or that it has both truth-values. 'p', like any other statement in a presumably bivalent system, is either true or false. But to find out which is the case, we need to fix the context, which in this case is the truth-assignment to the propositional variable 'p'. –  Hunan Rostomyan Apr 25 at 11:01
    
Thanks for the insightful response. Here's another related question: can "I'm moving" be taken to be equivalent to "I'm moving with respect to some object"? It seems that the answer is "no", because in this case the statement would be a tautology (at least in our world where our planet revolves around the Sun). –  user132181 Apr 25 at 11:09
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It's not a tautology that one is moving with respect to some object. It's another implicit parameter. It can even be true if all things considered you are not moving. Motion is the shift in position relative to an observation point over time. –  virmaior Apr 25 at 11:15
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Thanks for the answers, guys :) –  user132181 Apr 25 at 12:00
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Ending a sentence with "moving" is ending it on a preposition, i.e. the sentence is incomplete and what you are moving in relation to is implied or (hopefully) obvious from context. So yes, it is a question of the language being ambiguous.

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I actually like this answer a lot :) –  user132181 Apr 25 at 14:46
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moving is not a preposition, it's a participle or noun derived from a participle (which may or may not be a gerund). –  virmaior Apr 26 at 1:20
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To me, it seems that we are talking of language being subjective.

"It looks as though the whole thing is a kind of a cheap trick, but what is it exactly?" I think it is the dynamic of language and communication. Language is a tool for communication which implies a certain degree of common understanding between the speaker(s)/transmitter(s) and the listener(s)/receiver(s).

In the case of "I'm moving", the listeners are assumed to use the Earth as the reference.

Isn't every single word an artificial construct? And, they could even mean different things in different languages. If what we call a "leaf" were to be renamed to "fael" tomorrow, would a leaf cease to be what it is?

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Very nice answer. –  user132181 Apr 28 at 13:41
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Thank you! It is my first post on this community. I feel encouraged to be active :-) –  Joseph B Apr 29 at 4:08
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