UPDATE (7/31): The following entry is deeply confused. But I will leave it up for the sake of the commenters, David Gordon and AJ, who refuted it. In my defense I will say something Roderick Chisholm once said about himself in a similar connection, namely, that I wrote something clear enough to be mistaken.
The following two sentences are in the active and passive voices, respectively:
1. Tom said that someone was in the vicinity.
2. Someone was said by Tom to be in the vicinity.
Both sentences 'say the same thing,' i.e., express the same proposition, the same thought, the same Fregean Gedanke. Aren't active-to-passive and passive-to-active transformations in general truth- and sense-preserving? But the two sentences have different entailments.
(2), which is de re, entails that someone was in the vicinity. (1), which is de dicto, does not entail that someone was in the vicinity. But if the two sentences have different entailments, then they cannot express one and the same proposition.
The puzzle expressed as an aporetic triad:
A. (1) and (2) express the same proposition.
B. (2) entails a proposition -- Someone was in the vicinity -- that is not entailed by (1).
C. If p, q are the same proposition, then for any proposition x, p entails x iff q entails x.
The limbs of the triad are individually plausible but collectively inconsistent.
How do we solve, or perhaps dissolve, this puzzle?