Raising and Controlargument cannot be realised in the two different subject positions as there is only one -
role involved. A verb like want, on the other hand, can realise arguments overtly in both
subject positions as there are two independent -roles. So, one kind of missing subject
shares a -role with another element in the sentence while the other kind of missing
subject has a -role all of its own. The next three examples demonstrate that the two
different kinds of missing subject have different referential properties. The non-
independent type of missing subject which shares a -role with its antecedent, must be
lower in the structure than its antecedent. Hence it cannot be part of a structure which is
raised to a higher position and the ungrammaticality of (47a) follows. The independent
type of missing subject on the other hand can, under certain circumstances be higher in
the structure than its antecedent, hence the grammaticality of (47b). The contrast in (48)
again shows a difference in the referential properties of the two missing subjects. In
(48a) we see that it is impossible for the dependent missing subject to refer to an object:
they are always associated with subjects. In (48b) we have an independent missing
subject, it being the one who is doing the looking after. As we can see, it is capable of
referring to the object as the ultimate meaning is that Larry will be the one ‘looking after
himself’. Finally, (49) shows that the dependent type of missing subject cannot refer out
of the subject clause of another clause, where as the independent missing subject can.
Summarising, the dependent type of missing subject shares a -role with its
antecedent and is fairly restricted in its referential properties, always being below its
antecedent which is a subject in the immediately higher clause. The independent
missing subject has its own theta role and demonstrates far more flexible referential
properties. Its antecedent can be a subject or object, higher or lower, close by or more
distant, given the right circumstances. If the missing subjects in both cases were the
result of an absence rather than a presence of something unpronounced, it would be
rather difficult to account for these differences. If there really is something in these
positions, then the evidence suggests that there they come in different types which
have different properties.
Of course, the fact that the dependent missing subject shares a -role with its
antecedent can lead to only one conclusion, given the theory of -role assignment
discussed in chapter 2: the missing subject and its antecedent are a single argument. In
other words, this kind of phenomenon is the result of movement and the missing
subject is a trace:
(51) D-structure e seems [Tim to be tall]
S-structure Tim 1 seems [t 1 to be tall]
In reality then, the subject is not ‘missing’, it has just moved. By the same conditions,
we cannot treat the independent missing subject as the result of movement: it bears a
-role different from its antecedent and so they do not represent a single argument, but
two different ones. Moreover, at D-structure these two arguments must also be in
different positions as different -roles are assigned to different positions at D-
structure. Hence we conclude that this kind of missing subject is present at D-structure.
As we see in (51), this is not true of the other ‘missing’ subject. Because this element
has many properties in common with a pronoun, i.e. it bears an independent -role but
can be referentially dependent on something else, it is often referred to as PRO.
In the next sections we will look at these two different elements individually.