For Mann, electronic instruments, i.e. electrophones, are a proper subset of quintephones, but the category quintephone is necessary to describe computational sound synthesizers that operate by other-than-electrical means, such as synthesizers that work using optical computing.
The classification has still not been debated in organological studies.
Categories of quintephone 
- Electrophones, i.e. electronic instruments;
- Synthesizers based on optical computing;
- Synthesizers based on mechanical computing;
- Synthesizers based on life forms, neural networks, etc.;
- Physiologically generated music, e.g. electroencephalophone or electrocardiophone.
The name derives from Plato's and Aristotle's "fifth" Classical Element after Earth (solids), Water (liquids), Air (gases), and Fire (plasmas). This fifth element, which they called "Idea" or quintessence (from "quint" meaning "fifth"), describes that which is beyond the material ("matter") world and its four states-of-matter. Quint is Latin for "fifth" and "phone" is Greek for "sound", so an alternative (all-Greek) name would be "pemptophone" (all Greek), but "quintephone" is used because the fifth Greek Classical Element is often now more commonly known and described in terms of the Latin "Quint" as "Quintessence" rather than "Pemptousia" (Greek for "fifth essence").
Instruments belonging to the fifth Hornbostel-Sachs category, "electrophones", added by Sachs in the 1940s (Kartomi, book reference) are quintephones, but a number of other instruments have been invented that synthesize sound, or record and play back sound samples, optically, mechanically, or otherwise. Some quintephones work entirely without using electricity in any way, and are thus certainly not electrophones, yet they generate sound computationally rather than acoustically.
Comparison with other instruments 
Instruments generate sound either acoustically (from matter in its solid, liquid, gaseous, or higher-energy state), or informatically, from matter in its state of Quintessence (Quintessence, also known as Idea, was the fifth-classical element of Plato and Aristotle).
Quintessence-based sound production 
Instruments that generate sound from quintessence (Idea) use some form of computation, algorithm, or calculative process, whether by analog circuits (as in the Theremin), by digital circuits (as in modern software synthesizers), by mechanical computing (as in the use of the phonograph disk as a sampling instrument), or by optical means (as in instruments like the Optigan).
- Electronic instruments generate sound through electronic means. They often mimic other instruments in their design, particularly keyboards, drums and guitars. Examples: synthesizers and theremins.
- Mechanical computation, synthesis, or sampling instruments
Instruments like the turntable generate sound mechanically, although they record specific samples. Other similar instruments have been built that use mechanical computing rather than electronic computing in order to achieve sound synthesis, storage and recall of sound samples, and mechanical manipulation of sound samples.
- Sound synthesis using optical computation, optical sampling, optical storage, and the like. Instruments like the Optigan use optical storage media. Other similar musical instruments have been made from motion picture film projectors that have an optical sound track.
- Sound production by neural networks. Sound can also be produced by a neural network such as the human brain. This sound can be brought out raw (as in a performance at ICMC 2007) or can be post-processed by passing it through various pitch transposers, and even using it to control other instruments as was done in the DECONcert series ["DECONcert", Proceedings of the International Computer Music Conference, August 2007, Copenhagen, Danmark].
Further reading 
- Mann, Steve. "Natural Interfaces for Musical Expression: Physiphones and a physics-based organology".