[…] if opening is the action, and the participants a gardener and a gate , I may choose to say 'the gardener opens the gate' or 'the gate is opened by the gardener'. We will have to distinguish between an active function, and a passive function, these functions being, of course, nothing but our traditional ‘voices’. It is not easy to tell what functional indicators are in 'opens' and 'is opened'; probably '–s' for one function, 'is…ed' for the other.
- Martinet (1962), a pag.49-50