Difference between "provided that" and "unless"?

Question

Do these two sentences mean the same?

1. Nothing is an offence which is done by a person who, at the time of doing it, is, by reason of intoxication, incapable of knowing the nature of the act, or that he is doing what is either wrong, or contrary to law: provided that the thing which intoxicated him was administered to him without his knowledge or against his will.

2. Nothing is an offence which is done by a person who, at the time of doing it, is, by reason of intoxication, incapable of knowing the nature of the act, or that he is doing what is either wrong, or contrary to law: unless that the thing which intoxicated him was administered to him without his knowledge or against his will.

Answer 1

The multiple negations of various sorts make it a little difficult to track what is going on in the Original Poster's examples. That's quite normal; it's what multiple negations do.

The second sentence of the Original Poster's does not mean the same as the first. Indeed, it would be a very strange state of affairs if the second sentence were true. The meaning is along the following lines:

• Generally, if a person is too drunk to know what they are doing or to know that it's illegal, they aren't committing a crime. However, if someone spikes their drink without them knowing and they do the same thing, they are committing a crime.

So in such a case, if I gleefully drank a bottle of whisky and then stole a police car and drove it home, I would not be committing a crime. But in contrast, if someone put a load of vodka in my pint of lemonade and I didn't know about it, and I stole a policeman's car and drove it home, I would have committed a crime.

The first example, however, says what we might expect, that:

• People are not committing a crime when they're too drunk to know what they're doing - if someone spikes their drink.

This first example implies, but does not state, that people are committing a crime if they knowingly drink so much that they don't know that what they're doing is illegal.

Q, provided P is logically equivalent to Q, if P. In contrast, Q, unless P is equivalent to Q, if not P.

Answer 2

In any pair of sentences of the form

A provided B

and

A unless B

the two have essentially opposite polarities.

Compare Walking in the street will be safe provided the ice melts and Walking across the pond will be safe unless the ice melts. The former asserts that the ice’s melting is a sufficient condition for safety, whereas the latter asserts that it’s a sufficient condition for danger.

Answer 3

Let...	denote...
X(a)	Act a is an offence.
Y(a,p,s)	Act a is done by person p while they are, due to intoxication by substance s, incapable of knowing either the nature of the act or that the act is wrong or contrary to law.
Z(p,s)	Person p was administered substance s without their knowledge or against their will.

1. 1. Nothing is an offence which is done by a person who, at the time of doing it, is, by reason of intoxication, incapable of knowing the nature of the act, or that he is doing what is either wrong, or contrary to law: provided that the thing which intoxicated him was administered to him without his knowledge or against his will.

   The phrase ‘provided that’ just means ‘if’. Thus, the above verbiage is apparently saying

   • if   Z   then   ( if Y then not X ).

   Equivalently:

   • if   Y and Z   then   not X.

   In other words:

   • if an act is done by a person while they are, due to intoxication by a substance administered to them without their knowledge or against their will, incapable of knowing either the nature of the act or that the act is wrong or contrary to law, then the act is not an offence.

   This law sounds reasonable.

2. 2. Nothing is an offence which is done by a person who, at the time of doing it, is, by reason of intoxication, incapable of knowing the nature of the act, or that he is doing what is either wrong, or contrary to law: unless that the thing which intoxicated him was administered to him without his knowledge or against his will.

   The word ‘unless’ literally means ‘if not’.^҂ Thus, the above verbiage is apparently saying

   • if   not Z   then   ( if Y then not X ).

   Equivalently:

   • if   Y and not Z   then   not X.

   In other words:

   • if an act is done by a person while they are, due to intoxication by a substance administered to them with their knowledge and not against their will, incapable of knowing either the nature of the act or that the act is wrong or contrary to law, then the act is not an offence.

   This law is very lenient!

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^҂Supplementary note

The word ‘unless’ literally means ‘if not’. For example, the observation “in the 1940s, unless she wore gloves, a woman wasn't considered properly dressed” does not additionally claim that every glove-wearing woman in the 1940s was considered properly dressed, and the directive “don't compliment unless you mean it” carries no instruction to give a compliment whenever one means it.

As such, ‘W unless Z’ can be paraphrased as ‘W and Z are not both false’, which is equivalent to ‘at least one of W and Z is true’, which is equivalent to ‘W or Z’, where the connective ‘or’ is in the inclusive sense; in other words, the relation ‘unless’ is, strictly speaking, a disjunction. (In practice though, the relation ‘unless’ is frequently intended to convey the exclusive disjunction—in other words, for ‘W unless Z’ to mean ‘W or Z but not both’.)

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Reply to comment

@ryang - So Collins Dictionary is wrong in your eyes?

@KateBunting  Collins is correct. Referring to the statement

• B unless A,

your linked Collins definition

• use ‘unless’ to introduce {the only circumstance} in which [a statement you are making is not true]

says that {A being true} is the only circumstance in which [B is not true]; in other words, if B is not true, then A must be true; that is, ‘A if not B’, which is actually equivalent to ‘B if not A’. QED.

(However, in the Usage Note further down the page, Collins says

• ‘unless’ is used to introduce {the only situation} in which [something will be true]

instead, contradicting its main definition of the word. Here, “will be true” is of course a typo that ought to be corrected to “is not true”.)