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Source: Continuous indicates duration without interruption. ...

Continual indicates duration that continues over a long period of time, but with intervals of interruption. ...

The adverbs continuously and continually preserve the same distinction:

I already understand, and so ask NOT, about the foregoing confusables. Instead, my problem is: I always mistake one for the other. I heed the Etymological Fallacy,
but why does -ous => WITHOUT interruption, and -al => WITH interruption?

2. Is there a lesson in the big picture? What are some formal terms describing this issue?
Are there any other pairs of words with the same root, also liable to such a muddle?

  • Why are people continually asking this question? – Hot Licks Mar 25 '15 at 17:51
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WARNING: This answer responds to the question in the OP's header, "Why does 'continual' involve interruption, and 'continuous' none?" as a question about the historical emergence of the two meanings. It does not attempt to answer the questions "why does -ous => WITHOUT interruption, and -al => WITH interruption?" and "Is there a lesson in the big picture? What are some formal terms describing this issue? Are there any other pairs of words with the same root, also liable to such a muddle? " which the OP asks in the body of the question. If the header question isn't the one you're interested in, please don't waste your time reading the rest of this very long answer. Thanks!


Merriam-Webster's Webster's Dictionary of English Usage (1989) devotes almost three full pages of double-columned type to examining the history of the distinctions between continual and continuous and between continually and continuously. Its conclusion: the main distinction insisted upon by many grammar and usage commentators today—between uninterrupted occurrence (continuous) and repeatedly stopped and restarted occurrence (continual)—did not arise organically from the usage patterns of everyday speakers and writers of English, but was instead imposed by usage commentators seeking to govern actual usage by fiat. WDEU's lengthy entry for "continual, continuous" begins with this paragraph:

continual, continuous As far as we can tell, the first person to draw a distinction between continual and continuous was Elizabeth Jane Whately, in her book A Collection of English Synonyms, published under her father's name in 1851.

Her father was Richard Whately, Archbishop of Dublin. An 1873 edition of the book—which has the curious feature of being named A Selection of English Synonymes on its title page, but A Collection of English Synonyms on all of the running heads throughout the book—offers this entry for the words continual, continuous, and perpetual:

CONTINUAL, CONTINUOUS, PERPETUAL.

A 'continuous' action is one which is uninterrupted, and go on unceasingly as long as it lasts, though that time may be longer or shorter. 'Continual' is that which is constantly renewed and recurring, though it may be interrupted as frequently as it is renewed. A storm of wing or rain, which never intermits an instant, is 'continuous;' a succession of showers is 'continual.' 'If I am exposed to continual interruptions, I cannot pursue a continuous train of thought.'

'Perpetual' is sometimes used in the sense of 'continual,' but has rather a stronger signification, implying something which is still more constantly recurring. It also means something which is at once continuing and lasting; as 'the perpetual motion.'

Whately's discussion of perpetual usefully enlarges the scope of the examination of continual and continuous and hints at the larger field of words that apply to things that go on with or without intermission—a field that also includes incessant, unceasing, ceaseless, unrelieved, unremitting, constant, persistent, sustained, uninterrupted, unbroken, nonstop, recurring, recurrent, periodic, cyclical, regular, intermittent, repeated, sporadic, discontinuous, scattered, and irregular. Under the circumstances, Whately's attempt to draw a bright line between continuous and continual may have been motivated less by an arrogant desire to dictate the terms of proper English usage to everyone else than by a public-spirited desire to establish a workable distinction between words that many people were prone to using interchangeably. Or it might have represented an acute early observation of a contemporaneous sea-change in usage that soon led to altered definitions of continual and continuous.

Anyway, back to the WDEU's coverage of the two words:

Continual is the older word, dating from the 14th century. The definition given first in the OED encompasses both the sense Miss Whately prescribes for continuous and the one she prescribes for continual: the latter is marked "less strictly" in the OED. Continuous came along in the 17th century and was first applied to continuity in space. The earliest distinction between the words may be the one made by Dr. Johnson in his 1755 dictionary: "Continual is used of time, and continuous of place."

Oddly, I can't find the statement "Continual is used of time, and continuous of place," which WDEU cites from the 1755 dictionary, anywhere in the 1756 two-vulume edition of Johnson's dictionary (I ran searches for the phrase "continual is used" in both Volume I and Volume II). Here are the relevant definitions in Johnson's Dictionary of the English Language (1756):

CONTINUAL. a. {continuus, Latin.} 1. Incessant ; proceeding without interruption. [Authority:] Pope. 2. {In law.} A continual claim is made from time to time, within every year and day. [Authority:] Cowel.

...

CONTINUOUS. a. {continuus, Latin.} Joined together without the intervention of any space. [Authority:] Newton.

Historically, in discussions of the nature of space, the notion of continuous space (in effect, space without spaces) was viewed as being in opposition not to continual space but to discrete space (space composed of or filled with particulate matter).

As late as the 1847 edition of Webster's An American Dictionary of the English Language (which Merriam-Webster continued to reprint as its standard current edition of Webster's until 1864), the Johnson definition of continuous—very slightly altered—was the only one it listed. Here are the 1847 Webster's entries for continual and continuous:

CONTINUAL a. ... 1. Proceeding without interruption or cessation ; unceasing ; not intermitting ; used in reference to time. [Example omitted.] 2. Very frequent ; often repeated ; [example omitted]. 3. Continual fever or continued fever ; a fever that abates, but never entirely intermits, till it comes to a crisis ; thus distinguished from remitting and intermitting fever. 4. Continual claim ; in law, a claim that is made from time to time, within every year or day, to land or other estate, the possession of which can not be obtained without hazard. 5. Perpetual. [The same dictionary lists five definitions for perpetual.]

...

CONTINUOUS. a. ... Joined without intervening space ; as continuous depth. [Authority:] Thomson.

So Whately was introducing a major expansion of the notion of continuous—from application only to spatial continuity to continuity of thought and of phenomenological activity—including, presumably unbroken time. At the same time Whately was advocating that speakers and writers in English abandon their use of continual in precisely Webster's definition 1 sense of the word. That the efforts of Whately and her supporters succeeded at all is rather remarkable, given the way the two words were defined when her book appeared in 1851.

The 1864 edition of Merriam-Webster's American Dictionary of the English Language (which continued to be reissued as the current standard MW dictionary until 1890) alters the two main definitions of continual significantly, and greatly broadens the definition of continuous:

Continual, a. ... 1. Proceeding without interruption or cessation ; unceasing ; lasting ; abiding ; continuous. [Example omitted.] 2. Very frequent ; often repeated ; of frequent recurrence ; perpetual. [Example omitted.]

...

Continuous, a. ... 1. Without break, cessation, or interruption ; constantly prolonged ; protracted ; extended ; as, a continuous line of railroad ; a continuous current of electricity. "Continuous depth." [Authority:] Thomson. 2. {Bot.} Not deviating or varying from uniformity ; not varying. [Authority:] Henslow.

The 1864 Webster's then appends this very interesting usage note:

Syn. CONTINUOUS, CONTINUAL. Continuous is the stronger word, and denotes that the continuity or union of parts is absolute and uninterrupted; as, a continuous sheet of ice ; a continuous flow of argument. So Daniel Webster speaks of "a continuous and unbroken strain of the martial airs of England." Continual, in most cases, marks a close and unbroken succession of things, rather than absolute continuity. Thus we speak of continual showers, implying a repetition with occasional interruptions ; we speak of a person as liable to continual calls, or as subject to continual applications for aid, &c. To say, "It rained continually during the day," would not of necessity imply that there was no intermission whatever : to express that, we should be apt to use continuously. It is common to say, there are continual rains in the tropics at certain seasons. If continuous were used, it would imply that there was absolutely no cessation at all during the whole period, which is rarely, if ever, the case.

The change in the dictionary's handling of the two terms is striking. Without abandoning (or restricting the compass of) its first definition of continual as meaning "Proceeding without interruption or cessation ; unceasing ; lasting ; abiding ; continuous," Webster's decides that continuous is "the stronger word" to use in that sense—presumably because continually can also mean "Very frequent ; often repeated ; of frequent recurrence ; perpetual."

That someone writing as the Archbishop of Dublin could alter U.S. lexicographers' perceptions of two similar words so quickly and fundamentally is sufficiently remarkable that WDEU's concession to "Miss Whately"—

It is quite possible that Miss Whately was basing her discrimination on what she believed to be the cultivated practice of her time ...

—may not be concession enough. For WDEU, the two most important points are that many people continue to use continual in its original sense of "uninterruptedly" and to use continual and continually interchangeably in many situations. In making its case, it deemphasizes the significant and lasting changes in meaning of continual and (especially) continuous that occurred during the middle years of the nineteenth century—changes that Whately may have been observing and responding to when she formulated her distinction between the two words.

  • "Continual is the older word, dating from the 1th century." --- That's really old. – Greg Lee Mar 26 '15 at 0:02
  • @GregLee: Yes, that's back in the days of "1th upon a time." Thanks for alerting me to yet another example of my bad typing. – Sven Yargs Mar 26 '15 at 0:06
  • Are you sure it wasn't the Tooth Century? – Hot Licks Mar 26 '15 at 0:18
  • @HotLicks: It was certainly many decades before Tooth in Advertising regulations, as we know them today. – Sven Yargs Mar 26 '15 at 0:24
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According to the Grammarist, continual has an older origin (15th century) and originally meant without interruption. After continuous came into use (17th century) with that meaning, continual gradually had a sort of semantic shift and is now mainly used to mean with intervals. (note that the two terms are often used interchangeably)

Continual vs continuous:

  • Things that are unceasing or exist without interruption are continuous. For example, the flow of a river, the motion of the planets around the sun, and the heartbeat of a healthy human are continuous because they never pause. Things that occur frequently or recur intermittently are continual. The continual action doesn’t happen ceaselessly, but it does happen regularly. For example, phone calls to a busy office and departures from a bus station are continual because they happen regularly but not in an uninterrupted stream.

  • These definitions are only usually borne out in real-world usage. The words have not always been differentiated, and they are still often used interchangeably. When continual entered the language around 1400, it meant what continuous means now, and frequently occurring didn’t become its primary definition until the 20th century. Continuous entered the language much later—not until the 17th century—and its primary sense has always been uninterrupted or unceasing, but examples of its use in place of continual are easy to find.

  • Sorry, but would you please elucidate how this answers my question? I already know the difference; I instead ask about the semantic drifts that caused it. – Greek - Area 51 Proposal Aug 25 '15 at 18:37

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