The Agent is the external agency: in a material process it is the Actor, but only if there is a separate Goal; in a mental process it is the Phenomenon, provided the process is encoded in one direction, from phenomenon to consciousness (the 'please' type) and not the other way round […] In the attributive, this is a distinct function, the Attributor […] In the identifying, the function that is analogous to Agent is the Token, which is Subject in the active and 'by'-Adjunct in the passive [...].
- Halliday (1985), a pag.147-149