Exerpt from Possible Worlds. It is a pees of very useful information.
The method of possible-worlds testing is not only an invaluable aid towards resolving ambiguity; it is also an effective weapon against a particular form of-linguistic sophistry. Thinkers often deceive themselves and others into supposing that they have discovered a profound truth about the universe when all they have done is utter what we shall call a “Janus-faced sentence”. Janus, according to Roman mythology, was a god with two faces who was therefore able to ‘face’ in two directions at once. Thus, by a “Janus-faced sentence” we mean a sentence which, like “In the evolutionary struggle for existence just the fittest species survive”, faces in two directions. It is ambiguous insofar as it may be used to express a noncontingent proposition, e.g., that in the struggle for existence just the surviving species survive, and may also be used to express a contingent proposition, e.g., the generalization that just the physically strongest species survive.
If a token of such a sentence-type is used to express a noncontingently true proposition then, of course, the truth of that proposition is indisputable; but since, in that case, it is true in all possible worlds, it does not tell us anything distinctive about the actual world. If, on the other hand, a token of such a sentence-type is used to express a contingent proposition, then of course that proposition does tell us something quite distinctive about the actual world; but in that case its truth is far from indisputable. The sophistry lies in supposing that the indisputable credentials of the one proposition can be transferred to the other just by virtue of the fact that one sentence-token might be used to express one of these propositions and a different sentence-token of one and the same sentence-type might be used to express the other of these propositions. For by virtue of the necessary truth of one of these propositions, the truth of the other — the contingent one — can be made to seem indisputable, can be made to seem, that is, as if it “stands to reason” that it should be true. Among the more common examples of sentences which are often used in a Janus-faced manner is the sentence
(2.24) “Everyone acts selfishly all the time.”
It may be used to express the proposition
(2.25) No one’s acts are ever altruistic
in which case — on any ordinary understanding of what “altruistic” means — the claim being made is contingent but false. Or it may be used to express the proposition
(2.26) Every person’s acts are always performed by those persons themselves
in which case the proposition is undoubtedly true — because necessarily true — but is no longer an interesting topic for debate. The trouble is, of course, that someone may utter (2.24) with the intent of making a significant psychological claim about the sources and motives of human action — as in the manner of (2.25) — but, when challenged, try to save face by taking refuge in a tautology — such as (2.26). Not only is such a move on a par with crasser forms of prevarication; it may tempt us, if we do not keep our wits about us, to attribute to the contingent psychological claim the kind of indisputability which belongs only to necessary truths.
It should be evident how the method of possible-worlds testing can guard against sophistries of this kind. We need only ask the utterer of a token of a Janus-faced sentence-type whether there is any possible state of affairs in which the proposition being asserted is false. If the answer is “No” , then the proposition being asserted will undoubtedly be true, even though it may not strike us as very informative. But if the answer is “Yes”, then we shall want to enquire as to whether the set of circumstances in which it is false happens to include the actual world. All too often the contingent propositions which Janus-faced sentences may be used to express turn out not only to be possibly false but to be actually false as well.
Utterers of tokens of Janus-faced sentence-types may, of course, be quite unclear as to which kind of propositions they intend to express. Janus-faced sentences can beguile us all, speakers as well as hearers. But this much is clear: we cannot have it both ways; we cannot, that is, on one and the same occasion of the utterance of a token of a Janus-faced sentence-type claim both that it expresses a proposition possessing the indisputable credentials of a necessary truth and that it expresses a proposition which is distinctively true of the world in which we live. For no proposition is both contingent and noncontingent even though one and the same sentence-type may be instanced sometimes by tokens used to express a contingent proposition and sometimes by tokens used to express a noncontingent one.
For each of the following Janus-faced sentences explain how, on one interpretation, it may be used to express something indisputable (perhaps necessarily true), while, on another interpretation, it may be used to express something dubious (perhaps contingent and false).
1. “One cannot be certain of the truth of any contingent proposition.”
2. “Sounds exist only when they are heard.”
3. “All persons are born equal.”
4. “I can never have your thoughts.”
5. “The future must be what it is going to be.”
6. “Everyone is entitled to his/her own beliefs.”
7. “Tomorrow never comes.