That/Which

[P2052 at AOL.COM]

I've seen "that" used by people trying to avoid case problems, such as with
"who" and "whom."  I learned (I don't remember where) that the distinction
between "that" and "which" is not just one of context (restrictive vs
nonrestrictive) but also one of semantics.  For example, while both "who" and
"whom" can be either restrictive or nonrestrictive, case [position] is the
distinguishing feature (The former replaces subject pronouns whereas the
latter replaces object pronouns.)  With "which" and "that," however, the
opposite is true.  While both can occur in either subject or object position,
 "which" can occur in either restrictive or nonrestrictive contexts, unlike
relative "that," which can occur only in restrictive contexts.  A further
distinction is the semantic makeup of the referents.  "Which" can refer to
inanimate and animate non-human entities, whereas "that" can refer to
inanimate and animate (both human and nonhuman) entities.  Consequently,
"that" can be a substitute for "who," "whom," and "which," since its semantic
scope is much broader than that of the other relative pronouns.  It has,
thus, become an "escape hatch" or "dump" for those who don't care to remember
the rules governing the use of  relative pronouns.

Some (not very creative) examples are the following:
                 1.  This is the house [which/that] Jack built.
                 2.  This is the girl [who/that] lives next door.
                 3.  This is the man [whom/that] I plan to marry.
                 4.  ?This is that that I want to purchase.

                                                     PAT