Halliday (2008: 6):
The boundaries of any grammatical category are likely to be fuzzy […] — such indeterminacy is a general property of the grammar. The grammarian attempts to define each category as accurately as possible, looking at it from three different angles: its systemic environment (contrast with other term or terms in the system, and the relationship of that system to other systems); its meaning (proportionality in semantic terms), and its form. In other words, the grammarian adopts a “trinocular” perspective on the stratal hierarchy so that every category is viewed “from round about”, “from above” and “from below”. And since the views from these different angles often conflict, assigning instances to a particular category involves some degree of compromise, where criteria will depend on the purposes of the description.