Many readers will have spent the better part of their life hearing and reading the word 'unique' in a modified form. For example, a person might describe a situation as 'quite unique', or a music reviewer might praise an album as 'one of the most unique examples of [insert genre, technique, etc, here]'.
The problem is that you can't modify 'unique'. Or rather you can't always modify it, which leads to an interesting proposition.
The Word 'Unique'
Generally, any good English teacher will correct any modification of the word 'unique' on the grounds that being 'unique' is, if the reader will pardon the recursiveness, a unique state. In other words, an object/situation/person cannot be 'very unique', for example, because it is either unique or it isn't. Similarly, an object/situation/person cannot be said to be 'more unique' than another, simply because the state of uniqueness defies comparison.
Fine, then - you might say - so don't modify 'unique' and be done with it. However, it is possible to modify the word in a multitude of ways, none of which cause the problem described above. For example, one can describe a person as 'not at all unique,' or a situation as 'almost unique'. Since these descriptions do not contradict the uniqueness of the state of being unique, they do not create the same problem that phrases like 'sort of unique' do.
So what's the difference? One way to see it is in the form of a parallel to the study of numbers and mathematical limits.
The Number '0'
The number zero1 can be considered as the limit of any number of mathematical functions as a variable approaches another number. For example, the limit of the function x=x as x approaches 0 is 0. However, any other value of x will, obviously, not produce a number that is anything like 0. It might be close to 0, might be almost 0, but it will never actually be 0. This leads to the fact that you can describe a number as 'nearly zero', 'not at all zero' or 'almost zero', but never 'very zero' or 'somewhat zero' or 'more zero' than another number2. All statements lead to a logical fallacy.
By now the parallel should be clear. Replace the number zero for the word 'unique' in any English phrase and you should be able to clearly discern whether the modification is logically possible or logically impossible. A trivial result, perhaps, but a very satisfying one for anyone who has ever spent years struggling with this issue.
And now, as an exercise, spot the three (not four!) logical errors in the concluding sentences below.
I'm guessing that this is one of the more unique entries in the Guide, but I'm not sure just how unique it is. It's definitely not not unique; however, I hope to find that my next entries are considered especially unique.