A while ago, there was an answer on Jeopardy! along the following lines:

In the sentence he was on fire, the word on is this part of speech.

The judges ruled that it was a preposition. But I think that only works if the guy was physically located on top of a fire. After discussing it with someone, I think it's the first half of a compound adjective. There's a convenient parallel to the single word "afire" there.

So, who was right? And why?

  • 2
    Alec Trebec is never wrong!
    – OneProton
    Commented Aug 16, 2010 at 2:11
  • Bobby is on the phone. Is Bobby really sitting on the phone?
    – apaderno
    Commented Aug 21, 2010 at 0:55
  • @Atomix, who the heck is Alec Trebec?
    – Pops
    Commented Aug 28, 2010 at 15:46
  • Ah, apparently his name is spelled "Alex Trebek" I assumed that since he is Franco-Ontarian that his name was French. He's the host on Jeopardy.
    – OneProton
    Commented Aug 31, 2010 at 19:24
  • 1
    Do you think your question only works if that answer was physically located on top of Jeopardy!? Prepositions are pretty fuzzy anyway plus they can apply to abstract concepts just as well as physical ones plus they are just as subject to idiom as other parts of speech. But I also agree that it feels kinda wrong to me. In any case parts of speech are human invented classifications, they don't really have an essence. Commented Aug 23, 2011 at 18:46

4 Answers 4


The preposition "on" has a number different meanings, one of which is to "physically be on top of something". The preposition in the sense of "on fire" is one of them (from dictionary.com):

13 . in a state or condition of; in the process of: on strike; The house is on fire!

  • Hm... Merriam-Webster agrees. I suppose I'll accept this despite my English geek friends' protestations that this is a bastardization of the word.
    – Pops
    Commented Aug 28, 2010 at 15:48

"On" is a preposition, without a shadow of a doubt. "On fire" is a prepositional phrase which, like all prepositional phrases, can be used as an adjective or an adverb. (I'm trying to think of an adverbial usage for this one, and unfortunately I can't think of one.)


One cannot address the initial question without starting with an agreed definition of preposition.

A basic definition, which covers prototypical usage of members of the word class, and an accessible form of which is usually the first introduction to the subject given to children, is 'words used to show a spatial relationship between two referents' (eg The rabbit is in the box.). This is rapidly seen to be an unworkably narrow definition; there is a broadening to the identification of the use of prepositions in analogous directional (rather than just locative) constructions (Peter put the rabbit into the box.) and temporal constructions (Peter left after the match.). However, it has been found convenient to use analogous constructions in 'peripheral' cases - not involving space / time relations - and these can be considered to a lesser or greater degree metaphorical (with regard to the prototypical usages). The preposition now is defined from the perspective of grammatical function rather than from conceptual considerations, and this dual role causes confusion about the nature of the preposition. (See http://www.paaljapan.org/resources/proceedings/PAAL10/pdfs/kodachi.pdf)

'A preposition links nouns, pronouns and phrases to other words in a sentence' is the broader function-based definition / descriptor offered by by Heather MacFadyen - but this obviously isn't precise enough to exclude coordinators (eg and), complementisers (that), sentence connectors like additionally, and others. I prefer the requirement of 'within the same sentence', though - the claim for 'intransitive prepositions' (eg CGEL) is, I believe, ill-founded.(See http://stl.recherche.univ-lille3.fr/sitespersonnels/cappelle/Pdf%20versions%20of%20papers/The%20particularity%20of%20particles.pdf)

Prepositional phrases where the preposition involved seems 'arbitrary' are often, if not always, the result of changes over time in phraseology and/or what exactly was brought to mind when people first used the expressions compared with which, if any, picture is conjured nowadays. We are now left with (to us) unpredictable - idiosyncratic - set phrases. Idioms.

There is debate over the degree of 'decomposability' - how much value there is in analysing individual words - for various idioms. (See Fixed Expressions and Idioms in English: A Corpus-Based Approach by Rosamund Moon.) There was a school of thought that perhaps over-emphasised the 'word-with-spaces' analysis (so ship of the desert = camel, each of which denotes a single referent described by a single lexeme, no matter whether this uses 1 word or 4). This approach has value (but does not give the complete picture). Using this model, as kiamlaluno suggests, on fire (= afire) would be treated as a single (two-word) lexeme corresponding to a (predicate-only) adjective. (Where collocation ends and compounding begins is a different matter!)

At http://grammar.ccc.commnet.edu/grammar/prepositions.htm is the statement 'Prepositions are sometimes so firmly wedded to other words that they have practically become one word.' Admittedly, the authors are there referring to many (noun etc + preposition) constructions, but the statement is equally true for some (preposition + noun etc) constructions, as here.


It seems that the assignment of 'spatial' prepositions (on, under, into, above, etc.) to abstract situations is entirely arbitrary and varies from language to language. Within English it varies, too. She was on drugs and under the influence.

  • 1
    ... in an altered state of consciousness. Commented Aug 1, 2012 at 12:44
  • 1
    ... and totally out of it. Commented Nov 7, 2013 at 23:04

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