Propositional attitude

A propositional attitude is a mental state held by an agent toward a proposition. 
Linguistically, propositional attitudes are denoted by a verb (e.g. "believed") governing an embedded "that" clause, for example, 'Sally believed that she had won'.
Propositional attitudes are often assumed to be the fundamental units of thought and their contents, being propositions, are true or false from the perspective of the person. An agent can have different propositional attitudes toward the same proposition (e.g., "S believes that her ice-cream is cold," and "S fears that her ice-cream is cold").
Propositional attitudes have directions of fit: some are meant to reflect the world, others to influence it.
One topic of central concern is the relation between the modalities of assertion and belief, perhaps with intention thrown in for good measure. For example, we frequently find ourselves faced with the question of whether or not a person's assertions conform to his or her beliefs. Discrepancies here can occur for many reasons, but when the departure of assertion from belief is intentional, we usually call that a "lie".
Other comparisons of multiple modalities that frequently arise are the relationships between belief and knowledge and the discrepancies that occur among observations, expectations, and intentions. Deviations of observations from expectations are commonly perceived as "surprises", phenomena that call for "explanations" to reduce the shock of amazement.
Issues.
In logic, the formal properties of verbs like "assert", "believe", "command", "consider", "deny", "doubt", "imagine", "judge", "know", "want", "wish", and a host of others that involve attitudes or intentions toward propositions are notorious for their recalcitrance to analysis.
(Quine 1956).
Indiscernibility of identicals.
One of the fundamental principles governing identity is that of "substitutivity", also known as fungibility — or, as it might well be called, that of "indiscernibility of identicals". It provides that, "given a true statement of identity, one of its two terms may be substituted for the other in any true statement and the result will be true". It is easy to find cases contrary to this principle. For example, the statements:
are true; however, replacement of the name 'Giorgione' by the name 'Barbarelli' turns (2) into the falsehood:
Quine's example here refers to Giorgio Barbarelli's sobriquet "Giorgione", an Italian name roughly glossed as "Big George." The basis of the paradox here is that while the two names signify the same individual (the meaning of the first statement), the names are not themselves identical; the second statement refers to an attribute (origin) that they do not share.
Overview.
What sort of name shall we give to verbs like 'believe' and 'wish' and so forth? I should be inclined to call them 'propositional verbs'. This is merely a suggested name for convenience, because they are verbs which have the "form" of relating an object to a proposition. As I have been explaining, that is not what they really do, but it is convenient to call them propositional verbs. Of course you might call them 'attitudes', but I should not like that because it is a psychological term, and although all the instances in our experience are psychological, there is no reason to suppose that all the verbs I am talking of are psychological. There is never any reason to suppose that sort of thing. (Russell 1918, 227).
What a proposition is, is one thing. How we feel about it, or how we regard it, is another. We can accept it, assert it, believe it, command it, contest it, declare it, deny it, doubt it, enjoin it, exclaim it, expect it. Different attitudes toward propositions are called "propositional attitudes", and they are also discussed under the headings of "intentionality" and "linguistic modality".
Many problematic situations in real life arise from the circumstance that many different propositions in many different modalities are in the air at once. In order to compare propositions of different colours and flavours, as it were, we have no basis for comparison but to examine the underlying propositions themselves. Thus we are brought back to matters of language and logic. Despite the name, propositional attitudes are not regarded as psychological attitudes proper, since the formal disciplines of linguistics and logic are concerned with nothing more concrete than what can be said in general about their formal properties and their patterns of interaction.