How does one define parts of speech?

Syntactic categories (parts of speech, POS) are defined in terms of four different kinds of criterion:

1. Form:
   1. Contexts (in which the members of the category occur with specific functions)
   2. Structure (components of members of the category and their functions)
2. Substance:
   1. Meaning (semantic interpretation of the members of the category)
   2. Surface (gestural / acoustic / visual interpretation of the members of the category)