Which is more certain - "sure" or "confident"?

They are synonyms, but that doesn't mean they are exactly the same.

According to Dictionary.com,

Sure, certain, confident, positive indicate full belief and trust that something is true. Sure, certain, and positive are often used interchangeably. Sure, the simplest and most general, expresses mere absence of doubt. Certain suggests that there are definite reasons that have freed one from doubt. Confident emphasizes the strength of the belief or the certainty of expectation felt. Positive implies emphatic certainty, which may even become overconfidence or dogmatism.

Unfortunately, almost no online dictionaries discuss connotations of words, or this would be more easily answered.

Sure and confident are very similar -- in fact, the first word in the first definition in The Free Dictionary (TFD) for sure is confident.

According to The Free Dictionary, in discussing the definition and synonyms of sure:

definition of sure: Confident, as of something awaited or expected: I am sure we will win the game.

As a synonym for sure, TFD says:

Confident suggests assurance founded on faith or reliance in oneself or in others: "It goes without saying that a smiling, confident person will do better in an interview than a surly one" (Barbara Ehrenreich)

Many -- not all -- of the quotations from literature given by The Free Dictionary support this nuance. For many quotations, see TFD,sure; and TFD, confident

Jane Austen, Pride and Prejudice I know of nobody that is coming, I am sure, unless Charlotte Lucas should happen to call in-- And I hope MY dinners are good enough for her.

Upton Sinclair, The Jungle: Jurgis was confident of his ability to get work for himself, unassisted by any one.

How does this answer the OP's question? Confident suggests a basis for the expectation -- one's own work or abilities or a close knowledge of someone else's abilities. Sure has a weaker basis -- for example, the speaker in Pride and Prejudice merely expected that no one was coming. Thus, confident is stronger -- although confidence can be misplaced. Sure is weaker -- although it may turn out to be correct.

Well here are my thoughts. I think confident sounds more on conviction side. Confidence is something which you get from your own inner strengths and abilities while sure is more dependent on external factors. For example we usually say are you confident enough to pursue this goal. Here it is asking the person to say yes or no on the basis of his/her own ability. While if we ask are you sure this technique will work (to solve a problem) we say on the basis of what we have known from others' (and sometimes our own) experiences like yes it worked in case of my friend so I'm sure it will work for me as well.

Others have given the dictionary definitions of sure and confident, so I'll just proceed with an answer based on usage.

If someone says, "I'm sure I left it here somewhere.", the literal meaning is that they are confirming its location. However, in popular usage, it conveys uncertainty, so much so that it is the subject of an internet meme. It is also common to come across statements like "I don't remember any of it but I'm sure that's how it happened."

On the other hand, "I'm confident that's how it happened." is a statement that doesn't convey uncertainty.

Compare the google searches for the following phrases: - "don't remember but I'm sure" yields about 1,300,000 results; but - "don't remember but I'm certain" yields none.

My friend and I have an ongoing debate over which word communicates a stronger sense of conviction. ... Is this debate even resolvable?

According to the above, confident communicates a stronger sense of conviction than sure in the context of your sample sentence.

You are asking which word is stronger, which assumes that they are points on a continuum. What you will find is that both represent a range, and those ranges overlap. It's like asking which is greater, a number in the range 4-7 or a number in the range 5-9? Or even comparing 5-7 with 4-9, where one range is entirely within the other. Each person is likely to have their own idea of where in the range each word should fit. As such, just about any answer is correct.

• sure is stronger than certain
• sure is weaker than certain
• sure is usually stronger than certain
• sure is equivalent to certain
• etc.