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I already know, and so ask NOT about, the meanings or proofs of: A unless B = A, except if B
= A if not B = Either A or B. I ask only for intuition; do NOT prove formally or use truth tables.

My problem: I want to avoid memorisation. So whenever I see unless, I always need to pause for 5 minutes to rewrite the sentence, in order to determine the meaning. This pause reveals deficiency in my knowledge, which I must resolve, because it smothers me.
So please help me dig deeper. How can this conjunction be naturalised?

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    It can be used in more than one way. If you could give specific examples of uses that have confused you and required a lot of thinking on your part, that would be helpful... similarly, if you could post examples of uses of unless you understand, that too can be helpful to us.
    – Catija ♦
    May 3, 2015 at 22:54
  • And all that the Lorax left here in this mess was a small pile of rocks, with one word... "UNLESS." Whatever that meant, well, I just couldn't guess. [...] "But now," says the Once-ler, "Now that you're here, the word of the Lorax seems perfectly clear. UNLESS someone like you cares a whole awful lot, nothing is going to get better. It's not. [...] --The Lorax by Dr. Seuss (Here is Ben Rogge's performance on YouTube)
    – Damkerng T.
    May 3, 2015 at 23:19
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    Unless = "except if"
    – StoneyB on hiatus
    May 3, 2015 at 23:51
  • @StoneyB Thanks. Am I right that you found that definition from ODO? I included it above, but even so, how can I overcome my difficulty above?
    – user8712
    May 4, 2015 at 1:48
  • Are you clear that "A" and "B" here are conventional notations for "A is true" and "B is true"? That is, "A unless B" means "A is [necessarily] true, except if B is true (in which case we don't know whether A is true or not)"
    – StoneyB on hiatus
    May 4, 2015 at 1:57

3 Answers 3

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Unless is a negated if. Like if, it forms parts of conditional expressions.

I will go if John goes.

I will go unless John goes = I will not go if John goes.

Note that you aren't supposed to use then with unless (it's often optional with if), though you may hear it anyway in speech.

If the weather is good, then we'll go to the park.

Unless the weather is bad, we'll go to the park.

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It might help to think of it as:

"I want to go to a restaurant" -> Medium priority

"Unless you're feeling ill" -> High priority

Now there's a queue of things that will effect what I will do. If you're ill, that is more important than my desire to go to a restaurant. Otherwise, there's nothing stopping me from going.

This way of thinking should hopefully stop you from having to mentally go back and change A and B to be dependent on each other. You won't need to think "A is false if B is true", A is always true but will be overruled by B if B is present.

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Here's a mnenomic that may help you remember the meaning. Unless is a word formed from a prepositional phrase in Middle English of|in|on lesse than... It refers to a sufficient condition. (The mnemonic: less <=> sufficiency).

Don't bring the huge umbrella unless it's pouring rain.

A downpour is reason enough to take the huge umbrella.

P.S. Per request, sufficient.

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  • +1. Thanks. Would you please explain the mnemonic behind less <=> sufficiency? How do these 2 relate?
    – user8712
    Jun 10, 2015 at 22:23
  • Would you please respond in your answer, which is easier to read than comments?
    – user8712
    Jun 10, 2015 at 22:24

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