what does "if" mean?

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  • What does "if" mean?

    Kazuyoshi KAMIYAMA2017/2/14

  • CONTENTS

    Introduction

    Conditionals

    The Interpretation of if in logic

    A philosophical problem of conditionals

    Possible worlds approach

    The suppositional theory

    Remark

    References

  • INTRODUCTION

    Edington(2014) is a survey on indicative conditionals. In the survey

    she seems to assume that conditional sentences have only one

    analysis. She supports the non-propositional analysis. In this slide I

    assert that conditional statements have three different uses. In other

    words I propose a hybrid analysis of indicative conditionals.

  • CONDITIONALS

    If p, then q ,q if p are conditional sentences(in short,

    conditionals).

    The base angle is equal if triangular ABC is an

    isosceles triangle.

    We'll be home by ten if the train is on time.

  • If p, then q

    pthe antecedent

    q: the consequent

  • From linguistic point of view there are two types of conditional

    statements.

    A. Indicative conditionals

    Ex. (*) You become healthy if you eat an apple every morning.

    A. Subjunctive conditionals (or counterfactual conditionals)

    Ex. (**) "If you ate an apple every morning, you would be

    healthy."

  • THE INTERPRETATION OF IF IN LOGIC

    If p, the q (pq) is true except in the case in

    which p is true and q is false.

    The if interpreted like this is called the material

    implication.

  • Material implication assigns truth values to conditional

    sentences as follows;

    If Tokyo is the capital of Japan, Tokyo is in East Asia. true

    If 3>1, then 3>5. false

    If 3>1, then Tokyo is the capital of Japan. true

    If the moon is made of cheese, it is made of ketchup. true

  • A PHILOSOPHICAL PROBLEMOF CONDITIONALS

  • Indicative conditional

    (*) You become healthy if you eat an apple.

    Is this sentence true or false?

  • You eat an apple is an future event. So I cannot say

    that it is true at least now. I cannot say it is false either.

    The same can be said on "you become healthy."

  • As far as we interpret if as material implication, (*) is nonsense.

    Because p(you eat an apple) and q(you become healthy) are

    not true nor false. If cannot assign truth value to the whole

    sentence.

  • Subjunctive conditionals

    (**) "If you ate an apple, you would be healthy."

    (You did not eat an apple).

    If we understand if as material implication, (**) is true. Because

    you ate an apple is false.

    But,

    (***) "If you ate an apple, you would not be healthy.

    is true for the same reason.

    Logicians interpretation of if cannot discriminate (**) from (***).

  • It seems inappropriate to understand if in (*),(**) as the

    material implication.

    How should it be understood?

    (In this slide, we argue about only indicative conditionals.)

  • POSSIBLE WORLDS APPROACH

    (*) "If you touch that wire, you will get an electric shock".

    You don't touch it. Was my remark true or false? According tothe non-truth-functionalist, it depends on whether the wire islive or dead, on whether you are insulated, and so forth.

    Robert Stalnaker's (1968) account is of this type: consider apossible situation in which you touch the wire, and whichotherwise differs minimally from the actual situation. (*) istrue (false) according to whether or not you get a shock in thatpossible situation. (Edington,2014)

  • The possible worlds approach assumes the set of

    possible worlds. And it uses an ambiguous notion

    such as the possible situation otherwise differs

    minimally from the actual situation.

    If you do not want a heavy ontology and want to evade

    the ambiguous notion, you need to go to another

    approach-the suppositional theory.

  • THE SUPPOSITIONAL THEORY

    Along with the possible worlds approach, the suppositional

    theory(F.Ramsey(1929), E.Adams (1965) provides one of

    two basic answers to the above question. It goes as

    follows.

  • Let us ask what it is to believe, or to be more or less

    certain, that B if A -- that John cooked the dinner if

    Mary didn't, that you will recover if you have the

    operation, and so forth. How do you make such a

    judgement? You suppose (assume, hypothesise) that A,

    and make a hypothetical judgement about B, under

    the supposition that A, in the light of your other beliefs.

    (Edgington,2014)

  • Frank Ramsey(1929) put it like this:

    If two people are arguing "If p, will q?" and are both in

    doubt as to p, they are adding p hypothetically to their

    stock of knowledge, and arguing on that basis about

    q; ... they are fixing their degrees of belief in q given p.

    (Edgington,2014)

  • In this interpretation

    "If p, then q asserts that q is probably true under the supposition

    that p.

    In other words, the conditional probability of p given that q is high,

    i.e.,

    P(p/q) is high (for example, P(p/q) > 0.9).

  • This analysis seems natural. But, let us consider the

    following conditional statement;

    "If the 9th planet exists in the solar system, that will

    draw an elliptic orbit."

  • The X is the 9th planet in the solar system.

    All planets in the solar system draw elliptic orbits.

    The two sentences logically implies;

    The X draws an elliptic orbits.

  • In this case the following analysis is natural.

    "If p, then q asserts that the conjunction of p and the statements

    that we have accepted S implies that q. (p S q)

    * In this case a conditional statement is a shortened forms of

    logical implication.

  • So let me revise the above analysis:

    "If p, then q" says the following:

    1) the conjunction of p and the statements that we have

    accepted implies that q is true, or

    2) the conditional probability of q given p is high( for example,

    P(q/p) > 0.9).

    "If the 9th planet exists in the solar system, that will draw an

    elliptic orbit." is an example of 1) case.

    "We'll be home by ten if the train is on time is in the 2) case.

  • 2) case uses the notion of probability. Probability is interpreted

    in several ways. Basic interpretations are subjective one and

    objective one. In the former case probability means the degree

    of belief and in the latter case it means a physical propensity of

    an event.

    Note: Objective probability is interpreted as a physical

    propensity, or disposition, or tendency of a given type of

    physical situation to yield an outcome of a certain kind, or to

    yield a long run relative frequency of such an outcome. (the

    propensity interpretation of probability, See Propensity

    probability in Wikipedia)

  • Let us consider "We'll be home by ten if the train is on time.

    Well be home by ten could mean the objective probability

    that we are home by ten happens is high.

    So let us revise our analysis more.

  • "If p, then q" says the following:

    1) the conjunction of p and the statements that we have

    accepted implies that q is true, or

    2) the conditional probability of q given p is high( for example,

    P(q/p) > 0.9), where probability means objective one, or

    3) the conditional probability of q given p is high( for example,

    P(q/p) > 0.9), where probability means subjective one

    (degree of belief).

  • In the cases 1) and 2), "If p, then q expresses a proposition.

    Therefore we can give the truth condition to the conditional,

    which goes as follows;

    Case 1)

    "If p, then q" is true if and only if there exists a set of statements S

    that we have accepted as true and the conjunction of p and S

    implies q.

  • Case2)

    "If p, then q" is true if and only if the conditional probability of q

    given p is high( for example, P(q/p) > 0.9), where probability

    means objective one.

  • In the case 3) we cannot give a truth condition, because in this

    case "If p, then q" does not express a proposition, in other words,

    it does not claim truth.

  • REMARK

    The problem of compounds of conditionals (Edington,2014)

    If A, then if B, C"

    "It's not the case that if A, B

    "Either we'll have fish, if John arrives, or we'll have leftovers, if he

    doesn't"

    "If (B if A), C"

    If conditionals do not express propositions with truth conditions, we have no account of the

    behavior of compound sentences with conditionals as parts (Lewis (1976)). If we understand as

    above that conditionals express propositions, it is not the problem for the suppositional theory. In

    other words, if you use compound sentences, if means that in the case 1) or case 2) (if-sentence

    states a proposition).

  • REFERENCES

    Adams, E. W.(1965): A Logic of Conditionals, Inquiry, 8, 16697.Edgington,D. (2014): Indicative Conditionals,Stanford Encyclopedia of Philosophy

    (http://plato.stanford.edu/entries/conditionals/)Interpretations of Probability,

    (https://plato.stanford.edu/entries/probability-interpret/)

    Lewis, D.(1976): Probabilities of Conditionals and Conditional Probabilities,,

    Philosophical Review, 85: 297315

    Propensity probability(https://en.wikipedia.org/wiki/Propensity_probability)

    Ramsey, F. P. (19