„In his analysis of the meaning of declarative, jussive and interrogative sentences, Hare (1970) draws a valuable terminological distinction between what he calls the phrastic, the tropic and the neustic. By the phrastic he means that part of the setences which is common to corresponding declarative, jussive and interrogative sentences: its propositional content. The tropic is that part of the sentence which correlates with the kind of speech-act that the setence is characteristically used to perform: it is what Hare calls „a sign of mood“; and in many languages it will in fact be grammaticalized in the category of mood. The difference between the imperative and the indicative mood in Latin, for example, grammaticalizes the difference in the tropics of corresponding jussive and declarative sentences: e. g., ‘Dic mihi quid fecerit’ and ‘Dicis mihi quid fecerit’. The neustic is what Hare calls a „sign of subscription“ to the speech-act that is being performed: it is that part of the sentence which expresses the speaker’s commitment to the factuality, desiderability, etc., of the propositional content conveyed by the phrastic.

Like many authors, Hare frequently used the term ‘sentence’ where it would seem to be more appropriate to use the term ‘utterance’; nor does he distinguish clearly between ‘statement’, ‘declarative’ and ‘indicative’, between ‘command’, ‘jussive’ and ‘imperative’, and so on. We will treat the neustic, the tropic and the phrastic as being components of the logical structure of utterances.

Hare’s distinction of the neustic from the tropic separates two of the functions that Russel & Whitehead (1910: 9), folowing Frege (cf. Dummett, 1973: 308ff), ascribed to the assertion-sign (), which they prefixed to a propositional variable, in order to show that the proposition was being asserted as true, rather than merely being entertained or put forward for consideration. As far as straightforward statements of fact, or categorial assertions, are concerned, the tropic can be said to have the meanig „it is so“ and the neustik „I say so“. Both of these meanings are normally taken to be included in „it is the case that“ when we interpret the formulae of the proposicional calculus as having this phrase prefixed to them. But they can be dissociated. When a simple proposition (e. g., p) is embedded in a complex proposition (e. g., p ®  q), the I-say-so part of the assertion-sign („it is the case that“) is not applicable to the component simple proposition, but only to the complex proposition taken as a whole. The component simple proposition, however, still has associated with it what Hare calls a sign of mood („it is so“). When we make a hypothetical, rather than a categorical, assertion (e. g., If John is working, ...), we do not subscribe to the factuality of the proposition expressed by the embedded declarative sentence („John is working“); we nonetheless put this proposition forward for consideration as a fact, and thereby associate with it the it-is-so component of the act of assertion. Similarly, when we embed a declarative sentence as the object of a verb of saying in indirect discourse, we associate the it-is-so component, but not the I-say-so component, with the proposition that is expressed by the embedded sentence (cf. the statement He says that John is working).

The illocutionary force of a statement may be regarded as the product of its tropic and its neustic. As we shall see later, is in principle possible to draw a distinction between the unqualified assertion of the possibility of a proposition and the qualified assertion of its factuality; and this distinction can be handled in terms of the difference between qualifying the tropic and qualifying the neustic. English, however, does not systematically distinguish between these two kinds of modality; and perhaps no language does in primary performatives.

Mands differ fron statements in that their tropic is to be interpreted as „so be it“, rather than „it is so“. Whereas a statement tells the addressee that something is so, a mand tells the addressee that something is to be made so. Corresponding statements and mands can be said to have the same propositional content, but to differ in their tropic. Both categorial assertions and commands, however, contain the same unqualified I-say-so component, indicating that the speaker commits himself fully to the factuality (it-is-so) or desirability (so-be-it) of what is described by de phrastic. The difference of illocutionary force between categorial assertions and commands is, therefore, a function of the difference between „it is so“ and „so be it“.“ 

[Lyons, John: Semantics. Cambridge: Cambridge University Press, 1977, Bd. 2, p. 749-751]

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„En realidad, aunque por razones de claridad expositiva hayamos hablado de dos - o más - significados distintos en el verbo de la proposición principal cuando exigen indicativo o subjuntivo en la subordinada, no creemos que haya que recurrir a postular dos verbos creer, dos verbos pensar, dos verbos ver, etc., como se ha dicho alguna vez. El cambio de sentido que claramente se percibe y que para creer, por ejemplo, hemos descrito como ‘inclinación hacia el no, duda’ vs. ‘no adhesión’, quizá pueda ser satisfactoriamente explicado recurriendo a la distinción de Hare, adoptada por Lyons (Semantics, 2, Cambridge, CUP, 1977, pp. 749 y ss.) entre el frástico, el trópico, y el néustico de una locución. Si partimos de tres secuencias como

el frástico sería el contenido proposicional de cada secuencia, en este caso común a las tres: Petra salir de aquí: el trópico vendría a ser uno de los responsables de la modalidad de la secuencia, algo así como es el caso que (Petra salir de aquí) para (1) y (2), y que sea el caso que (Petra salir de aquí) para (3). Y el néustico el compromiso del hablante con que todo lo anterior es así o es deseable que así sea. De modo que una paráfrasis, sólo aproximada, naturalmente, de (1)-(3) quizá pudiera ser:

Quizá la distinción se entienda un poco mejor si decimos que una pregunta deliberativa [Entscheidungsfrage] como „¿Voy al cine o no?“ se podría parafrasear así:

„no estoy en condiciones de pronunciarme (néustico) sobre si es deseable que suceda (trópico) que yo vaya al cine (frástico)“.

Repárese en que „es posible que ciertos verbos lleven subjuntivo“ admite dos interpretaciones:

a.  Existe la posibilidad objetiva de que ciertos verbos lleven subjuntivo. Nada lo impide.

b.  Quizá ciertos verbos lleven subjuntivo. Yo no lo sé.

Pues bien, en a la posibilidad recae sobre el trópico, es objetiva, mientras que en b recae sobre el néustico, es subjetiva. Y si decimos „quizá exista la posibilidad de que ciertos verbos lleven subjuntivo“ hemos introducido la posibilidad tanto en el néustico como en el trópico (cfr. Lyons, Semantics, 2, 802 y ss.).

Volviendo a los verbos que nos ocupan, lo que parece suceder es lo siguiente: el significado ‘duda’ de no creer está ligado a la negación del néustico, y el significado de ‘no adhesión’ a la negación del trópico. Es decir:

La diferencia entre (7) y una pregunta como „¿Ha comprado flores?“ radicaría en una matización del néustico: en ninguno de los casos el hablante suscribe el contenido proposicional, pero en la pregunta no lo suscribe en absoluto, mientras que en (7) se inclina por suscribir que no es cierto, aunque manteniendo la duda.“

[Borrego, J., Asencio, J.G., Prieto, E.: El subjuntivo. Madrid,1985, p. 89-90]

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„Néustico

En la terminología del lógico Hare, las componentes semánticas de una proposición son la frástica (acerca del contenido proposicional), la trópica (acerca de la modalidad) y la néustica, es decir, la indicación que el hablante da de la propia intención, la parte en la que él afirma el grado del propio empeño de voluntad: ¡Que lo dejes en paz, te he dicho!“ [Cardona, G. R., p. 193]