In these matters the only certainty is that there is nothing certain.
Pliny the Elder

1. Ways of saying that you are certain about something

certain adjective if you are certain about something, you are absolutely sure that it is true. If a fact is certain, it is definitely true, or it will definitely happen:
• I am certain that his analysis is correct.
• It now seems certain that the earth’s climate is starting to change.
• One can never be absolutely certain of anything in science.
• The general addressed the troops before the battle, assuring them that victory was certain.
STUDY NOTE: Certain is more formal than sure. It also sounds more definite.
sure adjective [not before noun] if you are sure about something, you believe that it is definitely true or correct:
• I am sure that many other writers share this view.
• It is difficult to be absolutely sure about when the photograph was taken.
convinced adjective [not before noun] completely sure that something is true, especially when you cannot prove it but you have strong feelings about it:
• She was convinced that her husband was innocent.
• They became convinced of the need for better health education.
confident adjective [not before noun] sure that something good will happen, or that you will be able to achieve what you want:
• The researchers were confident of success. (=they were sure that they would succeed)
• In his report he said he was confident that standards would improve.
satisfied adjective [not before noun] sure that something is true, because you have enough information to make a decision:
• Professor Knowles is now fully satisfied that (=completely satisfied) the drug is safe for general use.
• The insurance company needed to be satisfied that the damage was caused by the storm.

Certainty & Uncertainty
have no doubt / be in no doubt to be so certain about something that there are no doubts in your mind:
• I have no doubt that a cure for the disease will one day be found.
• He said that he was in no doubt that the fire was started deliberately.
without (a) doubt used when you are completely sure about something:
• She was without doubt one of Mexico’s finest artists.
• His experience in London was, without doubt, very important to his career as a writer.
• Without a doubt, it is what we eat that determines our weight.
there is no doubt that (also there is no denying / disputing the fact that) used when you are completely sure about something:
• There is no doubt that violence on our TV screens does influence children.
• There is no denying the fact that these men are guilty of the most horrendous crimes.
• There is no disputing the fact that new technology has had a dramatic effect on our lives.
certainly / undoubtedly adverb used when you want to emphasize that you are certain of what you are saying:
• Businesses will certainly benefit from the new tax laws.
• Undoubtedly there is a link between smoking and lung cancer.
• Certainly it will be a long time before humans are able to live in other parts of the solar system.
• Beckham was the captain and undoubtedly the best known player in the England team.

2. Ways of saying that you are not certain about something

not certain / uncertain adjective [not before noun] not sure about something:
• I am not certain that the figures are accurate.
• People often feel uncertain about how to deal with this type of situation.
• It is uncertain whether his death was accidental.
• It is by no means certain that she is right. (=it is not at all certain)
STUDY NOTE: Grammar
Not certain/uncertain is more formal than not sure.
not sure / unsure adjective [not before noun] if you are not sure about something, you do not know exactly what it is, whether it is true, what to do etc:
• I am not sure whether this story is true or not.
• Police are still unsure about the precise details of what happened.
• Scientists are not entirely sure that (=not completely sure) life does exist in other parts of the universe.
have doubts if you have doubts about something, you are not sure if it is true, correct, or suitable:
• Many people have doubts about the methods used in the research.
• Doctors have doubts as to whether these alternative treatments really work.
be dubious about something to not feel sure that you should do something, because you can think of ways in which it could cause problems or go wrong:
• The writer is dubious about the effectiveness of prison as a way of dealing with drug related problems.
• Most medical schools are dubious about accepting students older than 30.

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