Principle A

Principle A is a statement in the government and binding theory of syntax that is intended to answer questions such as these: Why in sentences such as the following must 'Mary' and 'herself' refer to the same individual in the first case? Why isn't the second sentence grammatical? Why can't 'Mary' and 'herself' refer to the same individual in the second case?

Mary_i saw herself_i in the mirror
*Herself_i saw Mary_i in the mirror

Possible (wrong) easy explanations

Linear order: 'Mary' and 'herself' have to occur in left-to-right order. When they do, 'herself' takes 'Mary' as its antecedent.

The following show that this cannot be right. Even though 'Mary' and 'herself' are in the supposedly correct order, and 'father' is not a possible antecedent for 'herself', still the sentence is bad. Note also that on the most likely interpretation it seems to imply that Mary's father is female. This suggests that despite the implausibility of this interpretation, somehow 'herself' is taking 'Mary's father' as its antecedent. In the second sentence the two key terms appear in the supposedly correct order, but still something is amiss. What?

*Mary_i's father likes herself_i
*The book about Mary_i fell on herself_i

Subjecthood: Perhaps, we might think, it is critical that 'Mary' be the subject of the sentence. But even when 'Mary' is not the subject, there are grammatical sentences where 'Mary' and 'herself' successfully corefer.

John likes Mary_i's picture of herself_i

What sort of relationship?

How can we correctly describe the necessary relationship between anaphors (like 'herself') and their antecedents? To state the relationship we need to define several terms, the most critical of which is c-command.

Definitions:

c-command: Node X c-commands Node Y iff (if and only if) the first node dominating X also dominates Y.
co-indexing: Nodes are co-indexed if each carries the same subscript.
bound: An NP is bound iff it is co-indexed with a c-commanding NP.
free: Not bound.

Principle A applies to:

Mary_i saw herself_i in the mirror

      S
     / \
  NP     VP
  |    /    \
  N   V      NP
Mary saw    /  \
           N    PP
       herself  / \
               P   NP
              in  the mirror

John likes Mary_i's picture of herself_i

        S            
      /   \          
    NP     VP
   /      /  \
  N      V    NP
John  likes  / | \____
           NP  N      PP
           | picture /  \
           N        P   NP
         Mary's    of   | 
                        N
                      herself

*Mary_i's father likes herself_i

These are difficult to draw, so I switch to a different but logically equivalent form now...

(S
 (NP
  (NP
   (N Mary's))
  (N father))
 (VP
  (V likes)
  (NP
   (N herself))))

*Herself_i saw Mary_i in the mirror

(S
 (NP
  (N herself))
 (VP
  (V saw)
  (NP
   (N Mary))
  (PP
   (P in)
   (NP
    (Art the)
    (N mirror)))))

*The book about Mary_i fell on herself_i

(S
 (NP
  (Art the)
  (N book)
  (PP
   (P about)
   (NP
    (N Mary))))
 (VP
  (V fell)
  (PP
   (P on)
   (NP
    (N herself)))))

"An anaphor must be bound"

If this is correct, then it should predict which of the above sentences is grammatical and ungrammatical -- under the specified interpretations. We have a problem, however:

*Mary_i thinks the boy saw herself_i

(S
 (NP
  (N Mary))
 (VP
  (V thinks)
  (S
   (NP
    (Art the)
    (N boy))
   (VP
    (V saw)
    (NP
     (N herself))))))

This indicates that we must modify the principle as follows:

An anaphor must be bound in its own clause.

Note: Anaphors MUST have an antecedent in the sentence. If "herself" in the following sentence is taken to mean someone not mentioned in the sentence, the sentence is ungrammatical.

*Mary_i saw herself_j in the mirror

(S 
 (NP
  (N Maryi))
 (VP
  (V saw)
  (NP
   (N herselfj)
   (PP
    (P in)
    (NP
     (Art the)
     (N mirror))))))