To determine whether two sentences assert the same proposition:
1. Use techniques of classification and definition to identify
the concepts the words express. Two words express the same
concepts if they pick out the same class of things.
Jack is a Christian.
Jack is a Protestant.
Jack is a Baptist.
These three sentences express different propositions because the
terms in the predicate are at different levels of abstraction.
2. Ignore differences in connotation. Sometimes, words that
express the same concept have different connotations. They
convey different images or feelings, elicit different
associations in our minds, or express different attitudes.
Mary has a firm command of the subject matter.
Mary has a good comprehension of the subject matter.
These two propositions assert the same proposition. However, the
first conveys the image of power and control over the material,
whereas the second is more bland and doesn't convey an image at
3. Find a literal interpretation of all metaphors. To
determine how a given proposition is logically related to others, we
have to know exactly what the proposition does and doesn't say.
If two people are using metaphorical terms in an argument, we
won't know whether they are really talking about the same issue
until we formulate their positions in literal terms.
My love is like a red, red rose.
My love is beautiful.
These do not express the same proposition. "Beautiful" is a very
abstract word. The point of this metaphor is to convey the
particular kind of beauty the woman has: the dark and delicate, regal
beauty of a red rose distinct from other kinds of beauty.