I have always found the confusion among ensure, insure, and assure interesting.  Insure is easy—it has to do with buying insurance for something.  The way I keep the other two straight is that ensure is intransitive and assure is transitive.  My local Quality Assurance professional may assure me that my new car is good after ensuring that it is of high quality by testing it.

Unfortunately, today’s definition of ensure (to make sure) makes it sound transitive.

As I recall, a book that is probably gathering dust on my shelf at home, Fowler’s Guide to American English Usage, or something like that, had a multi-page discussion of these words.

This is not Fowler, but here’s what Merriam Webster’s Concise Dictionary of the English language says about this:

ensure, insure, assure Quite a few commentators insist on distinctions between these words, but Bernstein 1977 says there are none, flatly contradicting Einstein 1985, who says that each means something different.  Usage agrees better with Bernstein.  Here is what the synonymy paragraph in Merriam-Webster’s Collegiate Dictonary, Tenth Editon, says:

Ensure, insure, and assure are interchangeable in many contexts where they indicate the making certain or inevitable of an outcome, but insure sometimes stresses the taking of necessary measures beforehand, and assure distinctively implies the removal of doubt and suspense from a person’s mind.

And here are three typical examples:

... but the state has blocked the sale until Witco assures that any pollution on the property will be cleaned up - Barry Meier, Wall Street Jour., 7 Aug. 1985

... claims that his system will ensure iniformity in pronunciaton - Baron 1982

... held that school officials had the right to insure that a high-school assembly proceed in an orderly manner - William Safire, N.Y. Times, 24 Aug. 1986

A few commentators, such as Trimble 1975 and Sellers, 1975, suggest assure for people, ensure for things, and insure for money and guarantees (insurance).  These are nice distinctions, and you can follow them if you want to.  Assure is almost always used of people, in fact:

But I am writing to assure you no jury wouuld convict if you wanted to join me in murdering Eddy Duchin - Alexander Woollcott, letter, 22 Feb 1933

The rest of the recommendations rests on using ensure for general senses and reserving insure for financial senses.  This distinction has been urged at least since Folwer 1926, especially by British commentators.  It is in general true that insure is used for the financial uses.  However, both insure and ensure are used in general senses:

A solicitor is a man whose profession ensures that whenever you telephone him, he is in court - Alan Brien, Punch, 11 Sept. 1974

... so simple a thing as ensure that all third-grade teachers will be expert in spelling - Mitchell 1979

... would insure against any awkward second marriage - Mollie Hardwick, Emma, Lady Hamilton, 1969

... his sudden fame probably insure a backlash - Calvin Tomkins, New Yorker, Dec. 1982

Our most recent evidence shows that the distinction between ensure and insure is made more often in British written English than in American written English.

Brazilian dude

[quote author=bellator link=board=todays;num=1120835667;start=0#2 date=07/09/05 at 02:22:08]Hi Cary,

I hope the following is of assistance to you.

Thank you for your post, Brazilian_dude, which contains very useful references.

I agree that the meanings of to assure, to ensure, and to insure overlap. As is discussed below, the terms have an interesting history that explains how such confusion may have arisen. Cary is quite right in evaluating the matter as a muddle. This is evident at the outset, when one considers that, taken together, the etymologies suggest that the usage of the verb to assure had an influence on the meaning and usage of the term to ensure, and, further, that the verb to insure is a variant of to ensure, which replaced to assure, but only in the context of protecting against financial loss (insurance).

The following analysis describes the realtionship between the instant terms, and suggests how it may have come about.

VB (repost in order to edit)

[quote author=bellator link=board=todays;num=1120835667;start=0#3 date=07/09/05 at 02:39:47]TO ASSURE

Etymology
c.1375, from O.Fr. aseurer "to make sure", from V.L. assecurar, from L. ad- "to" + securus "safe, secure". 1533, "without care", from L. securus "without care, safe", from se cura, from se "free from" + cura "care". The verb is from 1593. The meaning firmly fixed (of material things) is from 1841, on notion of "affording grounds for confidence". "Security" is attested from 1432, from L. securitas, from securus

Discussion
The c.1841 meaning of to assure, namely, "affording grounds for confidence", is, in my opinion, the basis for the term’s current use by speakers who are attempting (in good faith, vel non) to foster a belief in the addressee that the speaker is sincere with respect to the putative reliability of his statement. In making his assurance, the speaker intends to influence the addressee’s state of mind, causing him to think in a specific manner about the outcome or result of some past or future event, or about the surrent state of something. The assurance is meant to make the addressee who is faced with uncertainty to feel "without care" and "safe" through a belief that an outcome or state of a certain situation will be favorable to him. An effective assurance affirmatively asserts that the assuring statement is reliable, and leads the addressee into assuming that the speaker’s statement is true. In making such an assumption, the addressee logically must believe: (1) that the speaker has personal knowledge, in adequate scope and depth, of the subject matter at issue; (2) that the basis for the speaker’s opinion is sound; and (3) that the speaker is objective, unbiased, and has no reason to misrepresent the truth.

In the fourteenth century, to assure was used as an affirmation that the speaker, himself, would endeavor, or cause others under his control to endeavor, to secure a favorable outcome for the addressee. In light of the fact that most transactions at that time were conducted in person, reliance on such affirmative statements was reasonable. A person’s reputation for truthfulness proved, a priori that he would act in good faith each time he gave an assurance.

Today, on the other hand, to assure is used simply to convince the addressee of the veracity of the speaker’s statement of assurance; in effect, to pursuade the addressee that the speaker’s prediction will come true or that his assessment is accurate. The modern meaning of to assure does not include the implication that the speaker will take any action. The phrase "I assure you that ... " puts the addressee on notice that the speaker, in fact, has little or no control over what will happen, or knowledge of the state of a situation. The assurance expresses merely the speaker’s opinion, and, as such, very well might be wrong. Think of the numerous times that we all of us have heard the encouraging assurances, such as "I am sure that the circus is coming to town", "I know that Tripping Tina will win by a nose in the fourth", and "There is no doubt in my mind that she will say ‘Yes’". It would be foolish to accept such conjecture as true, irrespective of our hopes, and no matter how emphatically the speaker assures us of such. Indeed, the rational among us do not rely on the speaker’s opinion, but, rather, continue to ruminate about the unknown.

In giving assurances, the speaker, at best, intends to express his point of view on the matter at hand in an attempt to alleviate the addressee’s concern. Even if the speaker knows nothing of the issue, and/or is powerless to affect the outcome of some event or does not possess sufficient knowledge of the state of some situation, he may, nonetheless, choose to assure the addressee, good-naturedly intending to help him to bear the tiresome waiting period before the outcome manifests or certitude is reached. At worst, the speaker offers his assurance with an intention to harm the addressee through deceit.

Less interesting, is when to assure is used not to bolster the addressee’s mood, induce reliance, or deceive, but, rather, merely to express an opinion on some matter in which the speaker and addressee have only a minimal interest, for example, "I assure you that it will not rain tomorrow".

VB (repost in order to edit)

[quote author=bellator link=board=todays;num=1120835667;start=0#4 date=07/09/05 at 02:51:04]TO ENSURE

Etymology
c.1385, from Anglo-Fr. enseurer, from en- "make" + O.Fr. seur "sure", probably infl. by O.Fr. asseurer "assure"

Discussion
While the meaning and usage of to ensure presumably was influenced by that of to assure, the term to ensure has surpassed the gravity of its forbearer by inheriting the strength of conviction of which to assure has been stripped. To ensure properly is used to express the speaker’s intention to make sure something will happen or to demonstrate that his knowledge of certain facts regarding an event or situation is sufficient to enable him to make a representation about the same with confidence.

A speaker’s well-founded belief that a certain event will ensue or that a given situation is in a certain state must be based upon: (1) his intention to perform certain acts himself to make something happen or affirmatively to confirm his belief about the state and/or nature of a given situation; (2) his expectations about and/or reliance on other people to do something; or (3) his expectations with respect to independent causalities. For example, an underling in a company who is addressing his boss may ensure that a report will be completed before nine o’clock the next morning. In doing so, he would be implying that he has some power to influence whether or not such will be the case. Upon hearing such a statement, the boss will sleep a little better than he would have if his employee had given him an assurance about the deadline, which would have expressed merely the worker’s opinion that the report timely will be submitted and that he has little or no control over the matter. If, on the other hand (see below), the junior executive were to insure that the report would be on his superior’s desk at the prescribed time, the boss would sleep very soundly, indeed.

To ensure is not "to guarantee", yet, it is interesting to note that the etymology of to ensure is related to that of to guaranty and the noun warrant. To guaranty means "to provide a guarantee serving to assure a particular outcome or condition". A warrant is "something that provides assurance or confirmation; a guarantee or proof: a warrant of authenticity; a warrant for success". In both definitions, to assure plays the role of "qualifier", in effect, diminishing the addressee’s certitude that the speaker will be capable of performing.

VB (repost in order to edit)

[quote author=bellator link=board=todays;num=1120835667;start=0#5 date=07/09/05 at 02:58:29]TO INSURE

Etymology
c.1440, variant of ensuren (see ensure). Took on its particular sense of "make safe against loss by payment of premiums" 1635 (replacing assure in that meaning).

Discussion
To insure now means "to make sure, certain, or secure.  With reference to the term’s etymology (which is intimately related to that of to assure, see above), the meaning of the term contains the important implication that the use of to insure necessarily requires that the speaker will "afford a guarantee". To be sure, a guarantee is affirmed when someone says, "I can insure that ... " To insure is the most concrete of assurances and it is only logical that it is used in the context of contractual obligations. For example, if a seller insures delivery on Tuesday, he means that he will "make certain" that it is so and that he will cure (make reparations) any problem, such as a delay in the shipment, that may arise with respect to the delivery. To insure, then, is to guaranty restitution of any losses incurred by the addressee that are related to the insured portion of the subject transaction. The reason that such firm responsibility inures to the speaker in the case of his usage of to insure is that parties to a transaction need insurance against the risk of loss. Unlike in the past, virtually all business transactions today are enforced by legally binding, written instruments. One likes to think of the "good old days", when a handshake constituted a guarantee (at that time, called an assurance). The increasing complexity that characterizes the progress of modern trade, however, has impacted the evolution of the term to insure, in that, previously, dealers reasonably could rely on each other’s perceived integrity and reputations. Unequivocal contract terms of assurance today are necessary in order for modern business to run smoothly.

VB (repost in order to edit)