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.”
Here are some 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).
Insure has come to mean “to protect oneself financially by insurance” (New Fowler’s 74)
An article on merriam-webster.com presents not only the fullest historical perspective on the trio, 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 – assure, ensure, insure – 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.