Incomparable…something that’s unequaled or matchless, or that’s beyond compare.
It’s the second definition that applies to incomparable adjectives (also called uncomparable adjectives): they describe an absolute state, so comparison is not possible. For example, dead is incomparable, because it describes an absolute condition. One corpse cannot be more dead or less dead than another: they’re both equally dead. Another obvious incomparable adjective is pregnant: a woman either is pregnant or is not; there are no intermediate states or comparative degrees.
Grammar guides, if they touch on this idea, typically provide a short list of easy-to-understand examples. Dead and pregnant, above, often make these lists. So do words like unique, perfect, blind, absolute, infinite, total, impossible, unanimous, ideal, and universal (as well as incomparable itself). It’s easy to find such lists, and once you understand the concept it’s not difficult to recognize others on your own: if an adjective has a simple binary state – true or false, left or right, right or wrong, on or off, trivial or important – it’s a good candidate for incomparable status.
Many guides will also point out that an adjective qualifies as incomparable if it cannot take a form that shows degree (a comparative or superlative), usually with the -er or -est suffixes: big is the base adjective (the positive degree); bigger is the comparative; biggest the superlative.
Things are rarely quite so cut-and-dried, especially in colloquial usage. It’s not uncommon to use additional modifiers with incomparable adjectives: the vote was nearly unanimous; the possibilities were almost infinite; acceptance was virtually universal; the victim was left partially blind in one eye.
People who care about this issue fall along a spectrum between two absolutes: all adjectives are incomparable, or none are.
At one end, it could be argued that all adjectives, if used properly, are incomparable. One strawberry is not redder than another: if one is red, the other must be a slightly different color; if, when the lights go out the room is dark, then we have to assume no light at all is present – there are no degrees of darkness.
At the other end, it could be argued that no adjective is incomparable. Simply because a modifier lacks natural comparative and superlative forms, or that its original meaning was excruciatingly precise, there’s no reason it can’t be modified: an idea can be more unique than another; one decision can be less catastrophic than another.
I’m deliberately staking out the extreme positions to make both arguments more clear, but very few people would make the case for either. It should be clear from my examples that both or these extreme positions are untenable.
So where is the sensible middle ground? Under most circumstances, you should probably continue to use adjectives the way you always have. But there are two things to consider.
You should be alert to those adjectives that either suggest (or demand) an absolute condition. Does it make sense to modify dead, pregnant, fatal, or inevitable? Can your context support it? Will your sentence sound silly if you modify that adjective?
You should also have a sense of your audience. Are you dealing with people who will insist that something is circular only if it’s in the shape of a circle? Or will your audience tolerate your description of something circular being less than perfect, perhaps only approximately round? Is the disease being discussed less communicable, or is communicable considered a binary state – it is or it isn’t?
In contemporary English, it seems obvious that the number of true potential incomparable adjectives is extremely small, only a dozen, perhaps two. But in practice even most of these can take modifiers without much trouble, conceptually or otherwise. One proposal can be more original than another; a woman in her third trimester is more pregnant than one in her first.
One can argue until they’re blue in the face that unique can only mean a singular item which can’t be compared to any other, and I would concede the argument on logical grounds. In actual use, however, I wouldn’t take issue if one dress were described as more unique than another. The idea that an adjective can’t be comparative or superlative merely because it doesn’t have -er or -est forms runs counter to the way English is actually used. I recognize the complaint that this is sloppy, or imprecise, and that it undermines the usefulness of words meant to have specific meanings. I recognize it, but I don’t entirely agree with it. This is, like so many other issues, a usage punch that contemporary English users need to roll with.
As a final note, when discussing incomparable adjectives perfect deserves a special mention. When people argue the idea of incomparables, they often cite the Preamble of the US Constitution:
We the people of the United States, in order to form a more perfect union, establish justice, ensure domestic tranquility, provide for the common defense, promote the general welfare and secure the blessings of liberty to ourselves and our posterity do ordain and establish this Constitution for the United States of America.
The use of more perfect is held up as a misuse of an incomparable adjective (something can’t be more or less perfect – it can only be perfect, an absolute state). It’s a good practical example in any argument for denying the strict use of incomparable adjectives. It needs to be pointed out, however, that this perfect uses the definition of complete or whole, not the definition of ideal. It’s the same sense used in perfect stranger, or perfect gentleman, or perfect storm: not some unparalleled form, but total or entire. That doesn’t change the incomparable argument, since all of those definitions are still covered. It just makes it a more interesting use.