Tema 27- Expresión de la certeza, la duda y la probablidad

Tema 27- Expresión de la certeza, la duda y la probablidad

Degrees of likelihood:

Instead of thinking of truth and falsehood in black-and-white terms, we can think of terms of likelihood. The extremes of the scale are impossibility and certainty or logical necessity. Other intermediate concepts to be considered are possibility, probability and improbability.

1. Impossibility

2. Improbability

3. Possibility

4. Probability

5. Certainty

These notions are expressed in various ways:

1. Most importantly by modal auxiliaries (can, may, must, could, might, will, would, shall, should)

– You may be right.

2. More formally by a sentence with introductory it and a that

– It is possible that you are right

3. By an adverbial such as necessarily, perhaps, probably, possibly, certainly

– Perhaps you are right

Auxiliaries such as can, may, must can refer to the future as well as to the present

– You may feel better tomorrow (it is possible that you will fell better tomorrow)

Possibility: CAN, MAY, COULD, MIGHT

1. Possiblity of fact (factual):

– The railways may be improved

– It is possible that the railways will be improved

– Perhaps/ Possibly/ Maybe the railways will be improved

2. Possiblity of the idea (theoretical):

– The railways can be improved

– It is possible for the railways to be improved

Theoretical possibility (can) is weaker than factual possibility (may). In the sentence with can the railways are improvable, that is, they are not perfect. With may, there are definite plans of improvement.

Can in general statements of possibility has roughly the same meaning as sometimes.

– Lightening can be dangerous

– Lightening is sometimes dangerous

Negation: For impossibility we use cannot or can´t (informal) but not may not

– He can´t be working at this time

– It is imposible that he is working at this time

– He is impossibly working at this time

In questions we use can not may

– Can he be working at this time?

– Is it possible that he is working at this time?

– Is he possibly working at this time?

Past time: For something which was possible in the past we use could

– In those days, a man could be sentenced to death for a small crime

For the present possibility of a past happening we use may + the perfect

– We may have made a mistake

– It is possible that we have made a mistake

For hypothetical possibility we use could or might.

– If someone were to make a mistake, the whole plan could or might be ruined

Tentative possibility: could and might in their hypothetical sense are often used to express tentative possibility that is to talk of something which is possible, but unlikely.

– He could/might be telling lies

– It is possible that he is telling lies


The notion of ability, also expressed by can, is closely related to that of theoiretical possibility.

– He can speak English fluently

Negation: for inability we use can´t

– He can´t speak German very well


– Can you drive a car?

Past time: COULD

– He could play the piano when he was five

Certainty or logical necessity: MUST, HAVE TO

– There must be some mistake

– You have to be joking (Am E)

– It is certain that the bombing will stop soon.

– Many people will certainly/necessarily lose their jobs

– Many people are certain/ sure to lose their jobs

– Inevitably, some changes will take place

There is a contrasting relation between possibility and certainty:

– His father can´t still be alive

– His father must be dead

– It is impossible that his father is still alive

– It is certain that his father is dead

All four sentences have in effect the same meaning

Negatives and questions:

– Does there have to be a motive for the crime?

– Is there necessarily a motive for the crime?

– Need there be a motive for the crime?

– Strikes don´t have to be caused by bad pay

– Strikes are not necessarily caused by bad pay

– Strikes need not be caused by bad pay

The auxiliary need is used in place of must in questions and negatives

Past time: We have to distinguish a past certainty (had to) from a certainty about the past (must + the perfect)

– Someone had to lose the game (it was necessary, by the rules of the game, for s.o. to lose)

– John must have missed his train (It is almost certain that John missed his train

Prediction and predictability: WILL, MUST

Must often expresses a certainty about an event which we do not observe, but about which we draw a conclusion from evidence.

On hearing the phone ring, you might say:

– That must be my wife

I know she is due to phone at about this time, and therefore I conclude that she is phoning now.

In a similar way, you can use will to make a prediction bout the present.

– That will be my wife

This sort of predication with will often occurs with conditional sentences

– If litmus paper is dipped in acid, it will turn red

Will can also express the idea of predictability or characteristic behaviour

– A lion will only attack a human being when it is hungry.

Its equivalent in the past is would

– He would often go all day without eating

Probability: OUGHT TO, SHOULD

The auxiliaries ought to and should can express probability and can be regarded as weaker equivalents of must (certainty)

– Our guests must be home by now (I´ m certain)

– Our guests ought to/should be home by now (They probably are but I´ m not certain)

Should is more frequent than ought to.

Other ways of expressing probability are:

– It is quite probable/likely that they didn´t receive the letter

– He is probably the best chess-player in the country

– They have very likely lost the way home.

– The concert is likely to finish late

Negation: Improbability can be expressed by shouldn´t, oughtn´t to or it is improbable/ unlikely that

– There shouldn´t be any difficulties

– There oughtn´t to be any difficulties

– It is improbable that there will be any difficulties

– It is unlikely that there will be any difficulties

In questions it is very rare:

– Should there be any difficulty in getting tickets?

Attitudes to truth:

We now consider the ways in which people may be committed or uncommitted to the truth or reality of sth. The people concerned may be the speaker “I” or another person or a group of people.

To express such attitudes we can use:

1. A that-clause or a wh- clause

2. Adverbials

3. The type of parenthetical clauses we call comment clauses: I know, I think, I´m afraid, I suppose, To be frank, As I said

1. Certainty:

– I know that his answer will be no

– I know what his answer will be

– I´ m certain/sure that the party will be a success

– The party will be a success, I´ m sure

– They were convinced that they would succeed/ of their success

– It is obvious/clear/plain that he has suffered a great deal

– He has clearly/obviously/plainly suffered a great deal

– We do not doubt that he is honest

– We have no doubt of his honesty

– Without doubt/ Doubtless/ Indubitably/ Undeniably/ Unquestionably, she is one of the best teachers in the school

2. Doubt or Uncertainty:

– I am not certain/ sure/ convinced that he deserves promotion

– I am not certain/ sure/ convinced whether he deserves promotion

– They were uncertain/ unsure (of) who was to blame

– I doubt if many people will come to the meeting

– I don´t think many people will come to the meeting

– There were some doubts about his honesty

– We have doubts about his honesty

– They were uncertain of/about the best course to take

3. Belief, opinion.

– I believe (that) the lecture was well-attended

– The lecture was well-attended, I believe

– He thinks (that) he can dictate to everybody

– It was everybody´ s opinion that the conference was a success

– It is my belief that cars will disappear from our roads by 1990

– In my opinion, he was driving the car too fast

– You may consider yourselves lucky

– He was thought/believed/considered to be the richest man in Europe

There is a slight difference between opinion and belief in that an opinion is usually sth. that s.o. arrives at on the basis of observation and judgement

– It is my belief that he drinks too much (I don´t know how much he drinks)

– It is my opinion that he drinks too much (I know how much he drinks, and in my judgement, it is too much)

Tag questions with a falling tone can sometimes be used to express an opinion

– He was driving too f`ast, w`asn’t he?

4. Assumption:

– We assume/suppose that you have received the package

– All the passengers, I presume, have been warned about the delay

– All the passengers have presumably been warned about the delay

5. Appearance:

– It seems/ appears to me that no one noticed his escape

– No one seems/ appears to have noticed his escape

– Apparently, no one noticed his escape

– It looks/seems as if you are right

– He looks as if he is ill

In that-clauses in the categories of opinion and belief, transferred negation is common. Instead of saying I think he hasn´ t arrived we prefer to say: I don´t think he has arrived.

In shortened reply statements, the clause which is the object of belief can usually be replaced by so:

– Hs the race been postponed?

– I think/suppose so

– It seems so

– I don´t think so

– I think not

Here so replaces “that the race has been postponed”.