Its logical structure is very different, however, from that of sense: sense gives acquaintance with particulars, and is thus a two-term relation in which the object can be named but not asserted, and is inherently incapable of truth or falsehood, whereas the observation of a complex fact, which may be suitably called perception, is not a two-term relation, but involves the Propositional form on the object-side, and gives knowledge of a truth, not mere acquaintance with a particular.
"Mysticism and Logic and Other Essays"
Bertrand Russell
The first point to notice is that, if any meaning is to be given to the phrase "would be true under all circumstances," the subject of it must be a Propositional function, not a proposition.
"Mysticism and Logic and Other Essays"
Bertrand Russell
Thus when it is worth saying that something "would be true under all circumstances," the something in question must be a Propositional function, i.
"Mysticism and Logic and Other Essays"
Bertrand Russell