These near-synonyms actually mean slightly different things—but teasing out the subtleties requires a bit of hairsplitting.

To drive the first wedge, distinguish between ability and capacity. Ability denotes actual (as opposed to potential) skill that may be either native or acquired.¹ On that point, Merriam-Webster’s Collegiate Dictionary cuts right to the pith, stating that ability is “natural aptitude or acquired proficiency.”^2(p3) Capacity, on the other hand, denotes the potential to develop a skill, a native characteristic that one either does or does not have and that cannot be acquired or developed.³ (Language purists might maintain that capacity should be used only to refer to space or volume, but its use to refer to aptitude is well accepted.) Moreover, whereas ability and capacity can each refer to either physical or mental aptitude, capacity is more commonly used in connection with mental aptitude—in particular, to “mental or intellectual receiving power; ability to take in impressions, ideas, knowledge.”⁴

Persons attempting to keep these subtleties straight might, like William Caxton, well be inclined to exclaim, “My capacity is not sufficient for the proper handling… of such subjects”⁴—but things get more complicated yet. When differentiating ability from capacity, some language users distinguish between whether a sentence is referring to persons/animals or to things (with ability often used with persons and capacity most often used with either persons or things)¹; however, this distinction is rapidly waning, and both words are commonly used to refer either to animate or to inanimate agents. For example, both words are commonly used when referring to inanimate agents such as physiological mechanisms (eg, “The ability/capacity of this pathway to promote….”) or anatomical structures (eg, “The ability/capacity of the liver to clear the body of these toxins….”).

Capability denotes “the quality or state of being capable”^2(p182) as well as “a feature or faculty capable of development.”^2(p182) Thus, capability comes closer to ability in meaning. However, capability further denotes a unique fitness for achieving a defined end,¹ and this specificity makes capability a good choice in contexts requiring a greater degree of precision. So, while capability is all too often simply used as a pretentious substitute for ability—a 10-dollar word lobbed in when a 1-dollar word would have gotten the job done—capability might have been the better choice in the above examples (“The capability of this pathway to promote….”; “The capability of the liver to clear the body of these toxins….”). Furthermore, language users often use capability in place of capacity, likely led astray by the “cap” with which they both begin. However, whereas the use of capability in place of ability is becoming more accepted, particularly when referring to a unique aptitude to accomplish a particular end, the use of capability in place of capacity is usually incorrect.

The bottom line:

Ability = Actual skill, either mental or physical; native or acquired.

Capacity = Potential to develop a skill, usually mental; native, as opposed to acquired.

Capability = Unique fitness for a defined end; sometimes may be used in place of ability, but its use in place of capacity is incorrect.—Phil Sefton, ELS

1. Ability, capacity, capability. Merriam-Webster’s Dictionary of Synonyms. Springfield, MA; Merriam-Webster Inc; 1984:4.

2. Merriam-Webster’s Collegiate Dictionary. 11th ed. Springfield, MA: Merriam-Webster Inc; 2003.

3. Ability, capacity. In: Bernstein TM. The Careful Writer: A Modern Guide to English Usage. New York, NY: Athaneum; 1985:5.

4. Capacity. The Compact Oxford English Dictionary. 2nd ed. Oxford, England: Oxford University Press; 1991:209.

Students are commonly taught that these words should be distinguished from one another, with aggravate used to mean “to make worse, more serious, or more severe”^1(p24) and irritate to mean “to provoke impatience, anger, or displeasure in.”^1(p663) However, aggravate has been used to mean irritate since at least 1611 (in a dictionary, no less: Randle Cotrave’s A Dictionairie of the French and English Tongues)² and moreover has been used in that sense by such writers as Cheever, Cowper, Dickens, Melville, and Styron.³

Despite that sterling track record, by 1870 the use of aggravate to mean irritate had for some reason begun to provoke finger-wagging,^1(p24) and currently such use is more acceptable in conversation and casual writing. (Perfectly understandable, really: who has time to stop and ponder which is correct, when one’s meaning seems clear enough using either word?) On the other hand, more formal writing—perhaps because the reader has the benefit of neither nonverbal cues nor personal acquaintance with the writer—often calls for more precision, and published writing commonly preserves the distinction between these words.^1(p24) Hence, many guides to written English continue to maintain that aggravate should never be used to mean irritate; for example, regarding such usage, Bernstein pointedly maintains that “neither the commonness nor the long history of misuse makes it any better than inept.”⁴

However, even writers who freely use aggravate in place of irritate do so only when describing a mental state—specifically, when someone or something is getting on one’s last nerve. Descriptions of physical states are another matter: while aggravate was used at least as early as the 1800s to indicate physical irritation (“With stinging wood smoke aggravating the eyes”),² precision mandates that that the distinction between the words be preserved in such contexts. For example, a patient with conjunctivitis does not have an “aggravation of the conjunctiva”; irritation, or a reaction to a stimulus, is the finding here, although the patient might be advised that environmental irritants such as smoke can further aggravate the irritation already present (see §11.1, Current and Preferred Usage of Common Words and Phrases, in the AMA Manual of Style, pp 381-405 in print).

The bottom line:

●Describing a physical finding or state? In casual as well as formal contexts, current usage calls for irritate, with aggravate used only to describe the worsening of the irritation.

●Describing the effects of irksome behavior or circumstances? The use of aggravate to mean irritate is usually acceptable in casual communications and is rapidly gaining acceptance in more formal contexts as well, but persons writing for publication might be wise to use aggravate only to refer to a worsening of one’s irritation. On this point, Bernstein again: “[t]hose who say they are aggravated are, most likely, the same persons who say that in the hospital they were diagnosed.”⁴—Phil Sefton, ELS

1. Merriam-Webster’s Collegiate Dictionary. 11th ed. Springfield, MA: Merriam-Webster Inc; 2003.

2. Aggravate. The Compact Oxford English Dictionary. 2nd ed. Oxford, England: Oxford University Press; 1991:28.

3. Aggravate. Merriam-Webster’s Dictionary of English Usage. Springfield, MA: Merriam-Webster Inc; 1994:49.

4. Aggravate. In: Bernstein TM. The Careful Writer: A Modern Guide to English Usage. New York, NY: Athaneum; 1985:30.

You’d think a nutritionist would know the correct usage of healthy vs healthful. Not so. At a recent nutrition educational meeting at the AMA, the nutritionist used the words interchangeably: healthful eating or healthy behavior. That’s a head-scratcher for me. I was hoping for clarification after a stinging rebuke—(I have been accused of being dramatic)—when my editing of healthy behavior to healthful behavior in a commentary had been reversed.

Shaken, I turned to the dictionary to confirm my position and found, as I thought, that healthful promotes health and healthy represents the state of good health. Yet, my editor’s complaint was that healthful seemed unnecessary. My attempts at persuasion fell short, so I had hoped the nutritionist’s discussion would help me gain my equilibrium.

Realizing her discussion placed her in my boat, I decided to see what the authorities who address it had to say.

Fowler’s Modern English Usage¹ notes that the distaste for the use of healthful is particular to the United States, a “problem that hardly arises in Britain.” Comparing 3 dictionaries for non-English speakers, Editor R. W. Burchfield observes that one calls its use “old fashioned or literary”; a second, “formal.” The third omits it.

Webster’s Dictionary of English Usage² says the confusion has existed since the 16th century and claims that the distinction between the terms was created as recently as 1881 by Alfred Ayers. Webster’s concludes that those who “observe the distinction between healthful and healthy … are absolutely correct, and in the minority.” Those who “ignore the distinction … are absolutely correct, and in the majority.”

The Chicago Manual³ hedges by making the distinction between the 2 words, but admits “But gradually healthy is taking over.”

As usual, it doesn’t really matter. But what should one do when making it an adverb? Healthfully or healthily? Hmmm.—Beverly Stewart, MSJ

1. Burchfield RW. Fowler’s Modern English Usage. 3rd rev ed. New York, NY: Oxford University Press; 2004.

2. Webster’s Dictionary of English Usage. Springfield, MA: Merriam-Webster Inc.

3. The Chicago Manual of Style. 16th ed. Chicago, IL: University of Chicago Press; 2010.