The notion of aspectuality has traditionally come to cover a number of distinct but closely related phenomena and approaches strictly intertwined with the notion of event and event structure. Consider the following examples:
|(1)||a.||? John loved Mary for/in three years|
|b.||John ran for/*in three hours|
|c.||John ate an apple *for/in three minutes|
|d.||John reached the top of the mountain *for/in three days|
It has been noted that these predicates differ as to their internal constitution and their temporal properties. Consider, for instance, (1a). There is a clear indication of a three-year period in which the predicate (to love) was true. However, it can be noted that much the same obtains for any subperiod of it. That is, for any subperiod t of the three-year period, the predicate to love can still truthfully apply. Similar considerations hold, at least to a certain extent, of activities. Surely, there are subperiods of a running that still qualify as running; that is, they show the subinterval property. On the other hand, predicates such as eating an apple do not display the same behaviour. If it took three minutes to eat an apple, as in (1c), then there is no subpart of this period to which the predicate eat an apple can be applied. In this respect achievements pattern with accomplishments. In the literature, these facts have been described by resorting to the notion of cumulativity and/or divisibility. Statives and activities are divisive whereas accomplishments and achievements are not. Other properties have been discovered, which are similar in nature to the one just discussed, e.g. summativity and so on. 2.2
The explanation of these facts crucially depends on a previous understanding of the ontological and semantic features of the involved entities, that is events. In this respect, two main approaches deserve consideration, the Montagovian one and the Davidsonian one.
According to the Montagovian tradition, events are properties of time, whereas Davidsonians regard them as primitive individuals in the ontology of natural languages. For a Montagovian, actionality properties are crucially explained by resorting to time entities. Thus, verbal predicates are true at time points or intervals, and a stative verb corresponds to a predicate which, whenever true at a time (interval) t, is also true at every subinterval included in t. All the other properties can be similarly reduced to properties of properties of times. For our purposes, it is important to notice that this way of explaining events require the full blown apparatus of higher order logic typical of the Montagovian tradition. This means that the approach might be of little value (at least in the majority of cases) for NLP, because of the well known problems of implementing such logics.
In the Davidsonian tradition, as elaborated by, e.g. [Hig85], [Hig97], [Kri89], [Kam93], not only events are primitive individuals, but event variables are introduced in the logical forms of sentences by verbs, nouns (as, e.g. in eventive nominalization) and so on. This paves the way for a rich, and relatively simple (basically first order) semantics of events, in which eventive variables can be quantified over by such devices as adverb of quantification (often, never, always, ecc.., see [Lew75], [Swa91]), modifiers directly attached to events (e.g., by conjunctive modification, as in [Dav67], [Hig85], [Hig97], and so on). In this respect, and partially independently from theoretical reasons, the Davidsonian approach recommends itself for NLP purposes, exactly for the reasons that made Montagovian approaches indigestible for NLP.
Turning to actionality-like properties in a Davidsonian framework,
they can be explained by directly resorting to properties of events, leaving
time in the background. One way of pursuing this approach consists in
singling out some structural primitive, e.g. part-of, which gives rise to
appropriate algebrical strucures (lattices and/or semi-lattices). The properties discussed
above can then be seen as properties of predicates defined on these
algebraic structures. This approach has been explored and made popular by
[Kri89], who follows similar suggestions for the objectual domain
advanced by [Lin83] and by previous work by [Bac86].2.1
One main point of Krifka's proposal concerns the existence of formal relations
between objectual and eventive domains (both conceived as algebraic
structures) which take the form of homeomorphisms. These relationships are
taken to be responsible for such contrasts as that in (2):
|(2)||a.||John ate an apple in/*for three minutes|
|b.||John ate apples *in/for three minutes|
Besides the merit of this, or other, particular theory, the relevant point is that actionality emerges not as a property of isolated verbs (or verbal stems) but, rather, as a compositional facet of complex structures. For our purposes, the morale is that verbal stems need to bear appropriate specification that can be used by a parser/generator and/or by a semantic processor to compute the correct actional value of a complex phrase, taking into account the properties of designated arguments.
latter point needs further clarification. In (2) the argument affecting the
actional properties of the (complex) predicate was the direct object. In
other cases, however, other complements/arguments can play a similar
role. This is the case of directional PPs with verbs of movement:
|(3)||a.||John ran for/*in two hours|
|b.||John ran home *for/in two hours|
|(4)||John walked around the fountain for/in ten minutes.|
As can be seen from the discussion above, actionality is the result
of the interaction between lexical properties (basically, aktionsarten of
the verb) and structural ones (the characteristics of the designated
argument/complement). Aspectuality, in the way this term is mostly used in
the literature, is a morphosyntactic property, deriving from specifications
borne by certain verbal forms, such as the Italian simple past, or from
free/bound morphemes that can (more or less) freely combine with the verb, as
in Slavonic and many other languages.2.3 The basic distinction is that
between perfectivity and imperfectivity. Interpretively, perfective verbal
forms refer to complete/finished events, as in:
|(4)||John ate an apple.|
|(5)||Mario mangia/mangiava una mela.|
|Mario eats/ate(IMPF) an apple.|
|(6)||Alle tre Mario scriveva la lettera e la sta ancora scrivendo.|
|At three, M. wrote(IMPF) the letter and he is still writing it.|
|(7)||Mario scriveva una lettera *per/*in tre ore.|
|M. wrote(IMPF) a letter for/in three hours.|
Also, teloses depend on the actional value of the complex
predicate. Thus, such predicates as eat an apple yield teloses, if
perfective. On the other hand, predicates such as eat apples, run,
etc.. remain non-telic even when perfective. In a sense, and for many
purposes, the telic-atelic distinction overrides and encompasses many
traditional aktionsarten ones, along the following schema:
|(8)||a.||Process & +SQA = telic|
|b.||Process & -SQA = atelic|
|(9)||a.||telicity & perfectivity +telos|
|b.||telicity & imperfectivity -telos|
To sum up. Actionality results from the combination of the lexical properties of the verb together with those of a designated argument. Aspectuality, on the other hand, is a morphosyntactic properties, depending on the presence of appropriate specifications bore by particular verbal forms or by bound and/or free morphemes attaching to the verb. Finally, telicity lies somehow in between actionality and perfectivity, in many respect depending on both. However, we saw that there is a class of verbs, the so called achievements, which are always telic and always actualise teloses.
Before concluding this sketchy introduction to actionality and aspectuality, one point must be stressed. First of all, actionality seems to be a cross-linguistically rather stable property. Thus corresponding verbs in different languages tend to have the same actional properties and much the same can be said about complex actionality. On the other hand, the perfective/imperfective distinction is highly language-dependent. This holds both with respect to the means available for expressing the distinction itself (complex verbal forms, as in Italian, French, etc., vs. free or bound morphemes attaching to the verb, as in Slavonic, etc.) and with respect to the range of possibilities available. Thus, [Gio97] argue that English does not have the perfective/imperfective distinction. In this language all eventive verbal forms are perfective. Similar conclusions can be drawn for many non-indoeuropean languages (Creole languages, African languages, Chinese, etc.).
In this section we will very briefly consider areas and phenomena which
overlap those discussed above. The first point to be addressed is the
progressive form (found in English, Italian, and, to a certain extent, in
other Romance and Germanic languages). Two different perspectives can be
taken. According to the first (see Giorgi and Pianesi, 1997)
the progressive should be distinguished from true imperfective forms. The former, in fact, has an intensional meaning that ordinary
imperfective forms lack.2.4 The
progressive should be regarded as a way to imperfectivise an otherwise
perfective form by intensionalising the completeness/finiteness of the
event. The distinction is subtle, but very clear empirically:
|(11)||John is reaching the top of the mountain.|
|Gianni sta raggiungendo la cima della montagna.|
|*Gianni raggiunge la cima della montagna.|
On the other hand, other scholars more or less tacitly assume that there is no real distinction between progressive and imperfective verbal forms, collapsing them under the same heading of imperfectivity.
As said above, the distinction is subtle and can probably be
neglected for many purposes. It cannot be ignored with such tasks as text or
dialogue generation and machine translation, though, at pain of producing a
wrong or ungrammatical form. So care must be taken in the design of the
basic aspectual classes for an NLP system. Another important overlapping area is constituted by all
such aspectual forms and values as inchoatives, resultatives and so on. It
should be noticed that many of the deviant examples discussed above can have
one of this readings. Thus (3a) becomes acceptable in following context:
(12) After the car accident, John ran in three days.
(12) has the inchoative meaning that John was able to run again only three days after the accident he suffered. We suggest that these cases be left unaddressed within this document. To be sure, the availability of such readings depends on a number of structural (syntactic) facts that are beyond the scope of our contribution.2.5 As such they should be treated by the syntactic and/or by the semantic analyser. In the case such aspectual values are borne by lexical items (aspectual verbs) or by free/bound morpheme, specifications similar to those we are going to propose for the basic aspectual opposition can be easily devised.
Most obviously, lexical aspect and actionality is connected with Semantic (or Thematic) Roles 2.4. For instance, within the approach advocated by [Kri89], the transfer of formal properties from the objectual domain to the eventive one depends on the nature of thematic relation involved, as analysed according to the suggestion in [Dow89] and [Dow91]. See 2.4 for a survey of the relevant problems.
Attempts have been made at capturing lexical aspect and aktionsarten within LDBs. An example of a static definition can be found in DIONYSUS [Nir93a] where aspectual specifications are provided which distinguish between phasal properties (e.g., begin), durational properties ( prolonged), and so on.
In ACQUILEX an attempt is made at a more dynamic classification of aktionsarten which also tries to address the interactions with thematic roles, [San91]. All this is done within a type feature structure (TFS) formalism. Thus, the traditional vendlerian distinction of verbal aktionsarten is reconstructed in the type hierarchy by means of the two types stative and dynamic. These are then combined with the two krifkean types cumulative and quantized to obtain a quadripartite classification of verbal predicates. Such a classification is then connected with the properties of thematic (proto-)roles to allow for the transfer of properties from the objectual to the eventive domain.
Concerning NL analysis in general, lexical aspect and actionality play a role in determining the grammatical and semantical interactions between predicates and other parts of the sentence, both arguments and modifiers. With respect to the former, besides the facts discussed in the previous sections, we want to mention genericity, habituality, event quantification, the mass/count distinction and so on, as cases in which the aspectual and actional properties of the predicate play a relevant role in determining syntactic and semantic facts. Concerning adjuncts, the actional properties of a (simple or complex) predicate determine the kind of temporal modifiers it can combine with (for- vs. in- adverbials), the kind of adjunct clauses (purpose and rationale clauses), etc. For all these reasons, lexical aspect and actionality play an important role in parsing (e.g., to restrict and control attachment ambiguities) and in semantic computations.
For the purposes of NL generation, and independently of the framework chosen to express the meaning of the text to be generated, the aspectual and actional properties of lexical items play a crucial role in lexical choice and in microplanning 4.4, 18.104.22.168 These considerations are even the more important when the system deals with multilingual generation. In such cases, in fact, stipulation of direct correspondences between actional and aspectual configurations and predicates on the basis of a single language inevitably fails to generalise to other languages, so that more principled approaches are necessary.
Concerning classes of applications, it is worth mentioning that because of the kind of (semantic) information provided, lexical aspect is important for all systems which must be able to detect and reason about events. This is true of information and knowledge extraction systems 4.2, dialogue management systems, etc... and in general for all the cases in which it is important to tell whether an action/event, as presented in the text or during the dialogue, is terminated/finished or whether it can continue at the speech time or at any other focalisation time. Furthermore, actionality is crucial to establish the appropriate relationships between the various events presented in the text/dialogue: simultaneity, overlapping, precedence, etc... Within machine translation 4.1, given the various means available to different langauges to express actionality and aspect, a proper mastering of these notions is crucial to improve the quality of the translation, [Dor93], by selecting/producing the appropriate constructions in the target language, etc., irrespective of the chosen approach (interlingua, transfer, mixed techniques based). Finally, we want to emphasise that the strict relationship between the eventual domain and the temporal one makes the aspectual information indispensable for systems performing any form of temporal reasoning: e.g., task scheduling, dialogue management, population and maintainance of temporal databases, etc..