In philosophy and in language there are two ways of reading a mention of something: it can be about the very thing itself (de re), or about the thing as described in the words used (de dicto). Most of the time these two readings coincide, but philosophically or linguistically interesting problems arise when they don't.

For example, Lois Lane doesn't know that Clark Kent can fly. Or does she? Superman can fly, she knows Superman can fly, and Clark Kent is Superman, so in a sense she does know Clark Kent can fly. That is, about the thing itself, de re, she knows Clark Kent can fly.

But about the choice of words used to name or describe or refer to this thing, she would not agree. Given the wording 'Clark Kent can fly', she would not say that. So de dicto she does not know he can fly.

The de re and de dicto interpretations of sentences typically diverge when someone is under a misapprehension or has imperfect information about facts or identities. The classic example, due to Gottlob Frege, is of the planet Venus, which the ancients called Phosphorus or the Morning Star when it was visible in the morning and Hesperus or the Evening Star when it was visible in the evening. They did not have to believe that Hesperus was Phosphorus. That is, they had a different sense (Sinn in Frege's terminology) for the names 'Hesperus' and 'Phosphorus' -- they thought of them differently, de dicto. But in reality, de re, they were the same, even if people didn't know this, and the names had the same reference (Bedeutung). So you could legitimately say and believe, perhaps, 'Hesperus must be further away than Phosphorus', without meaning the absurd 'Hesperus must be further away than Hesperus'.

The de re vs de dicto distinction is close to, but not identical to, the distinction between the referential and attributive uses of an expression. A referential use refers to some specific thing or person, regardless of what attributes it might happen to have; whereas an attributive use picks out whoever satisfies the property used to describe it. Attributive uses include 'whoever murdered Dr Black' and 'the winner of the jackpot' -- I don't know who did, only that someone did, and that's the person I'm talking about. But with referential uses I mean that very person there, and I might be picking them out with some property: for example, he seems to be drinking champagne, so I say 'the man drinking champagne', but I mean him. If it turns out he's drinking fizzy water out of a champagne glass I still mean him, and if unbeknownst to me someone else is surreptitiously drinking champagne, I don't mean that person: I was just using the attribute to pick out the object of my reference.

Why is de re not the same as referential, and de dicto not the same as attributive? Well suppose you and I don't know who murdered Dr Black, but the police do, and we learn that the police are keeping a close watch on Dr Black's murderer preparatory to an arrest. You and I have no referent we can refer to: all we can say is that Dr Black's murderer, whoever it is, is being watched by the police (attributive), but it is some definite person (de re), not just whoever they happen to be watching.

In Linguistics, de dicto/de re ambiguity is a logical issue in semantics and pragmatics.

Take the statement, "Jan wants to invite a clown to the wedding."

Assuming no contrastive stress, this has two readings:

1) the de dicto ("of what is said") reading - Jan wants to invite someone on the basis of them being a clown. He doesn't really care who the clown is, he just wants a clown to be present.

2) the de re ("of the thing") reading - Jan wants to invite an individual, say, Bozo, who happens to be a clown, and not necessarily because he's a clown.

These different readings are also represented differently in logic using predicate calculus.
W(x,Φ) - x wants to Φ where Φ is a proposition.
I(x,y) - x invites y to the wedding.
C(x) - x is a clown
j = Jan
And I will use E as the existential quantifier and subscript to represent my restrictions (I use restricted quantification.)

de dicto reading: W(j, ExC(x) I(j,x))
In English, roughly: Want holds between Jan and an Existant x, Clown of x, such that Invite holds between Jan and x.

de re reading: ExC(x) W(j, I(j, x))
English: There Exists an x, Clown of x, such that Want holds between Jan and x.
