120. The relative _what_ is handled differently, because it has
usually no antecedent, but is singular, neuter, third person. Its case
is determined exactly as that of other relatives. In the sentence,
"What can't be cured must be endured," the verb _must be endured_ is
the predicate of something. What must be endured? Answer, _What can't
be cured_. The whole expression is its subject. The word _what_,
however, is subject of the verb _can't be cured_, and hence is in the
nominative case.

"What we call nature is a certain self-regulated motion or change."
Here the subject of _is_, etc., is _what we call nature_; but of this,
_we_ is the subject, and _what_ is the direct object of the verb
_call_, so is in the objective case.

[Sidenote: _Another way._]

Some prefer another method of treatment. As shown by the following
sentences, _what_ is equivalent to _that which_:--

It has been said that "common souls pay with _what_ they do,
nobler souls with _that which_ they are."--EMERSON.

_That which_ is pleasant often appears under the name of evil;
and _what_ is disagreeable to nature is called good and
virtuous.--BURKE.

Hence some take _what_ as a double relative, and parse _that_ in the
first clause, and _which_ in the second clause; that is, "common
souls pay with _that_ [singular, object of _with_] _which_ [singular,
object of _do_] they do."