One of the concepts that is basic to the Western tradition of grammatical analysis is that of Subject [...] Various interpretations have grown up around the Subject notion, ascribing to it a number of rather different functions. These resolve themselves into three broad definitions, which could be summarized as follows: (i) that which is the concern of the message (ii) that of which something is being predicated (i.e. on which rests the thruth of the argument) (iii) the doer of the action These three definitions are obviously not synonymous; they are defining different concepts [...] When these [three] different functions came to be recognized by grammarians as distinct, they were at first labelled as if they were three different 'kinds' of Subject. It was still implied that there was some sort of a superordinate concept covering all three, a general notion of Subject of which they were the specific varities. The terms that came to be used in the second half of nineteenth century, when there was a renewal of interest in grammatical theory, were ʻpsychological Subjectʼ, ʻgrammatical Subjectʼ, and ʻlogical Subjectʼ.
- Halliday (1985), a pag.32-33