This trio – assure, ensure, insure – is another example of words that appear on many lists of words often confused. As with earlier pairs discussed, such as alright vs. all right and anxious vs. eager, the usage guidelines vary since, as merriam-webster.com asserts, “there is no unanimity of opinion as to what is correct” among commentators. Some believe that the confusion over the words is understandable, even justifiable, because they are similar and often interchangeable. Others, as with grammarly.com, insist that despite their similarity, “each of them has a distinct meaning that makes it better suited for some uses than the other two.”
The American Heritage Guide to Contemporary Usage and Style states that assure, ensure, and insure all mean “to make secure or certain” (43). The New Fowler’s Modern English Usage goes further to assert that “these three words have intersecting paths in contexts involving aspects of certainty, assuredness, and security” (74). In fact, in their various definitions listed in The Merriam-Webster Unabridged Dictionary, each of the words appear in definitions of the others words; while “each have some meanings which are peculiar to it,” they are not always discrete and “sometimes overlap almost entirely” (merriam-webster.com). This source points out that there are many times that the words are interchangeable, and this has been true since the development of the English language. In fact, “for hundreds of years insure and ensure were simply spelling variants, and had no more difference between them than theatre and theater” (merriam-webster.com).
However, with the movement to standardize the language, usage experts started in the middle of the 19th century to insist on distinguishing between these words to create what New Fowler’s calls “the main lines of usage” (74). Here are some of those traditional definitions.
Assure has been defined as “to give a guarantee to” (Line by Line 168) or “makes promises to, convinces” (Garner’s 69). For example, consider these sentences:
I assure you of my love.
I assured him that he had not been overlooked.
New Fowler’s would add the sense of to “be certain”: rest assured that I will be at the station when the train arrives (74).
Ensure is defined as to “make certain, guarantee” (New Fowler’s 74):
Checks at airports should ensure that no firearms are carried by passengers.
Our hosts ensured that we had comfortable rooms (Garner’s 69).
Though both assure and ensure have the sense of “to guarantee,” Garner’s would insist on the differences between the two and points to instances where assure frequently appears where ensure would be the better verb. For example, Garner’s quotes language expert William Safire’s misuse: “That would defeat the entire purpose of the legislation, which is to assure [read ensure] public perception of total independence.” (“See-Nothing Congress,” N.Y. Times, 23 June 1994, at A15).
Insure has come to mean “to protect oneself financially by insurance” (New Fowler’s 74). The American Heritage argues that ensure and insure are generally interchangeable, though noting that in American usage “only insure is now widely used . . . in the commercial sense of ‘to guarantee persons or property against risk’” (43). Garner’s is stricter in its usage advice, saying that insure should be restricted to refer only “to what insurance companies do,” while ensure “should be used in all other senses of the word” (69).
On the other hand, Claire Kehrwald Cook in Line by Line argues that either ensure or insure can mean “to make certain of.” She sums up the usage points on these three words in this way:
You can insure your possessions against fire and theft,
you can assure a prospective customer that no salesman will call, and
you can ensure or insure prompt delivery. (Line by Line 168-169)
However, Cook recommends that writers use one or the other consistently: “Some guides claim that insure is the more usual choice, but others would restrict insure to its narrow meaning, thus ensuring ensure the right to exist” (Line by Line 169).
We return to the article on merriam-webster.com, which presents not only the fullest historical perspective, but also a light-hearted approach, indirectly poking fun at purists and pendants. It poses these choices for writers considering which is the correct word of this trio to use:
“An optimist might view this as ‘no matter which one I pick some will think me correct.’
A pessimist will instead think ‘no matter which one I pick some will think me wrong.’
And a cynic will think ‘I do not believe that anyone truly cares about these matters, and therefore it makes not a whit of difference which one I choose.’”
Though there are of course people who do care, merriam-webster.com says that the cynic would be “mostly right” because of the history of the spelling variations and the interchangeable use.
As a writer, you need to decide what you think. The Everyday Writer notes, “Matters of usage, like other language choices you must make, depend on what your purpose is and on what is appropriate for a particular audience at a particular time” (299).
(Material adapted from merriam-webster.com; grammarly.com; The American Heritage Guide to Contemporary Usage and Style; The New Fowler’s Modern English Usage; Garner’s Modern American Usage; Claire Kehrwald Cook, Line by Line.)