1  Ayer on truth theories
2  The Poincare-Duhem problem

Chapter 5  is perhaps the philosophically richest chapter of Language, Truth, and Logic, so we will only be able to consider a handful of the important arguments Ayer gives.  His purpose in this chapter is to characterize synthetic sentences.


Ayer wants to argue that a certain view of truth undermines certain philosophical theorizing about truth.  To understand this we need some concept of what traditional truth theories tried to accomplish.  Philosophers prior to Ayer often thought that the proper explanatory task of philosophers was to provide a sweeping characterization of what truth was.  Here are cartoon versions of a couple of such theories.

For example Plato's theory of the forms counts as such a specification.  For Plato a proposition is true if the entity named by the subject instantiates the universal named by the predicate.

On this conception the philosopher needs to specify what this ``coherence'' relation comes to.   This is a difficult task, as it seems that whole groups of people can believe consistent, yet false beliefs.  For example, people used to believe that the sun orbited around the earth.  This agreed with everything else they believed, yet we now know the belief to be false.

On this conception the philosopher needs to specify what this ``usefulness'' relation comes to.  This is a difficult task, as it seems as that there are useful beliefs that are false (e.g. during the Inquisition it was useful for lots of people to believe in witches, as not believing in witches could get you killed by the Inquisition), and true beliefs that are not useful (for most people, knowledge of quantum physics is not very useful).   The American philosopher Charles Pierce tried to spell out usefulness in terms of scientific method.  Thus, he said that a proposition is true if, at a certain point in the ideal evolution of science, it comes to be accepted and is never subsequently rejected.

Ayer, I think, wants to argue that all such approaches are wrong.  In Chapter 5, he is defending a version of what is now called a ``redundancy'' account of truth.

On this view, the truth predicate doesn't really contribute anything to language.   Somehow, Ayer takes the Redundancy Thesis to make unsupportable the above three projects, and to support the project of asking under what conditions we are justified in asserting P.  What do we mean by ``the conditions under which we are justified in asserting P?''  This is a good question, one which Ayer is not totally clear about.  By our normal notion of being justified in asserting something we would take it to be the case that the following sentence is true.

For example, we may never be able to tell how old the universe is.  We may never be justified in the saying ``the universe is 15 billion years old,'' since we may never be able to know this is true.  For all this, ``the universe is 15 billion years old'' may be true.  Likewise, by our normal notion of being justified in asserting something, we also take the following to be true.

Consider past scientific theories that were entirely justified at the time people believed in them.  Some of these theories, such as Newtonian physics, are now known to be false.   Thus, Ayer's idea of replacing the notion of truth with that of being justified, or warranted, in asserting something seems not better off than the coherence and pragmatic theories of truth.

This is not the end of the story though.  Assume that one can replace the notion of truth with that of warranted assertibility (``P is warrantedly assertible'' is another way to say that one would be justified if one asserted P).  Even if one thinks that one needs to theorize about when we are justified in asserting things, as opposed to theorizing about truth, one still can reformulate the earlier truth theories as theories about the nature of evidential warrant.  Consider these:

Philosophical problems associated with truth don't disappear just because you stop talking about truth. Any debate between the classical truth theorists can simply be reformulated in terms of warranted assertibility.


Ayer has a certain picture of scientific explanation in mind that we can make more specific.   Consider this picture.

scientific laws       description of experimental setting      mathematical theory and approximations
          \                                   |                                                       /
           \                                 |                                                      /
             \                               |                                                     /
               \                             |                                                    /
                 \_____________ | _________________________/
                               prediction of some observation sentence

Ayer thinks of science as predicting observation sentences via logical relations.  An observation sentnece is any sentence which can be determined to be true or false simply by observation.  He seems to think of any scientific theory, that the laws of the theory, conjoined a with descriptions of an experimental setting, and perhaps some mathematical theory and approximations, will logically entail a predicted observational outcome of the experiment.   Thus, the laws of physics, along with descriptions of a lab experiment involving a ball and a ramp, plus some mathematics, entails a variety of measurable results concerning the ball rolling down the ramp.  Nearly every homework problem in a science class descrbes an experimental setting and asks you to determine the truth of some observation sentence (such as ``The ball is rolling at eight kilometers an hour 3 seconds after it is released'').  You have to figure out how to use the scientific laws and mathematics to logically derive the correct observation sentence.  The only difference between your homework assignment and what scientists do is that they go on to test the theory by seeing if the predicted observation sentence actually does turn out to be true when the experiment is run.

This picture of science gives rise to what people today call the ``Quine/Duhem hypothesis'' (though Jules Henri Poincare was the first to think of it).  The pre-Quine/Duhemian thought would be that if your theory predicted an observation sentence that was falsified by the experiment (say you predicted a dial in the laboratory would read 7, and it actually reads 5 when you do the experiment), then one of your scientific laws would be falsified. However if you look at the above schema you can see that it might have been either a faulty description of the experimental setting or faulty mathematics that caused you to come up with a false prediction.  Thus,  the scientific laws might be true, and the other theoretical apparatus involved in generating the prediction might be what went wrong.  Thus, when you get a falsified prediction, you have a choice, you are only forced to reject either the scientific laws, the description of the experiment, or the mathematics used.  The Quine-Duhem hypothesis, in effect, states a pessimistic conclusion about there being a fact of the matter about which of the three you have to do in such a situation.

The thought is that if we want to hold onto a belief badly enough we can always hold that belief true and make compensatory adjustments elsewhere to explain why our observations didn't accord with our predictions.

In a couple of extremely interesting passages (around p. 90-95) , Ayer argues that the Quine-Duhem hypothesis holds of sentences that describe observations.

Given the Quine-Duhem hypothesis, a central question concerns what good reasons there are for abandoning a hypothesis, and what experiments can be taken to confirm hypotheses. We can think of these questions in these ways: (1) When is an experimental result that doesn't jive with an experimental prediction taken to disconfirm a scientific law rather than disconfirm the description of the experimental apparatus or the mathematics used?  (2)  When do a set of experiments that do jive with the experimental predictions of a theory + experimental description + math provide good evidence that the theory is correct?

These issues are important because Ayer seems to want to treat all synthetic propositions in the way we may intuitively treat scientific hypotheses.  He states that the purpose of a synthetic proposition is to

This might seem intuitively plausible with scientific laws such as E = MC2.  Using Einstein's law, we can correctly predict the orbit of the planets, while the prior Newtonian equations generated false predictions for the orbit of Mercury (for example).

However, Ayer wants to say that all synthetic propositions have this predictive role, even such commonplace sentences such as ``Fido is a dog.''  There is something to this view.  For example if I believe that Fido is a dog, then I expect that in all of my future encounters with Fido, Fido will behave in characteristic dog-like ways.  The most important thing here is to realize that it is in virtue of the predictive function of sentences that they can be talked about as being verifiable.  If, every time I see Fido, he does characteristic dog-like things then the predictions that followed from my classifying Fido as a dog are verified.  If, however, Fido talks to me next time I see him, I would have evidence for the claim that Fido is not a dog.

When the predictions that follow from the assertion of a proposition are observed, then the probability of the propositions is increased.  Ayer holds, however, that the probability of a proposition holding can never attain 100%.  Further evidence may overturn the proposition.  On p. 101 Ayer writes that an increase in probability is an increase in ``the degree of confidence with which it is rational to entertain the hypothesis.''  These rational standards will be those which are used by scientists, not the ones we always use in practice.   This issue of which standards are correct bears directly on the Quine Duheim hypothesis; since it is only whole groups of sentences together that contradict observations.  When a prediction is refuted by evidence, we always need further standards other than logic to tell us which sentence in the set of sentences (that entailed the wrong prediction) to take as false.

While Ayer doesn't specifically discuss rational standards of science, a common rap about these standards starts by stressing certain theoretical virtues that good scientific theories have.  For example, some traditional virtues of scientific theories can be given thus:

Thus, even though falsified predictions do not uniquely tell us how to adjust our theory that gave rise to them, there are often good reasons that scientists have for picking one new theory rather than another.  Note- all of the above theoretical virtues are notoriously difficult to make precise sense of when looking at actual scientific theories.  As far as I know, no one has yet sucessfully characterized them in a manner which is non-vague and true to scientific practice.