Halliday & Matthiessen (1999: 190ff) provide a taxonomy of simple things based on the participant roles they play in semantic figures — critically those of Senser, Sayer and Actor.
The most general distinction is between conscious and non-conscious.
Within non-conscious, the distinction is between material and semiotic.
Within material, the distinctions are animal, object, substance and abstraction.
Within semiotic, the distinctions are institution, object and abstraction.
Material abstractions — eg depth, colours — typically play the roles of Phenomenon, Carrier and Value. They have no extension in space and are unbounded, and are typically some parameter of a material quality or process.
Semiotic abstractions — eg information, truth — are typically realised by the Range of mental and verbal processes. They are unbounded semiotic substance with no material existence.
There are also intermediate categories in this taxonomy. For example:
Human collectives — eg family — are intermediate between conscious beings and institutions.
Discrete semiotic abstractions — eg thoughts and fears (mental entities) and questions and orders (speech functions) — are intermediate between semiotic objects and non-discrete semiotic abstractions.