Why is it impossible to say how many types of synesthesia there are?

1. Types, subtypes, variants, manifestations, expressions… how should they be broken down? Is a “type of synesthesia” something very general like “auditory-visual”, or should more specific manifestations like “timbre-colour” also be included? “Time unit-colour” is considered a type (although it is really just a subtype of “coloured sequences”), but what about month-colour? Is that a type? And decade-colour?

There are numerous ways to subdivide them, and they could all be considered valid.

Each individual researcher or person who decided to make a list would use their own classification and the end result could be a vastly differing number of types.

Perhaps in future we will agree on an “official” method (or methods) of counting them.

2. A large number of very rare types of synesthesia have been documented, but what about a type that nobody actually has? Is that a type of synesthesia? Because there are many types that exist in theory but have never been reported, which means we are moving into purely theoretical territory. Morpheme-to-touch, anyone? Personality-to-sound? Pain personification?

3. And how should we classify the phenomena that are often considered a type but do not actually meet all the basic criteria for being synesthesia? Mirror touch, ticker tape and even ordinal linguistic personification raise some doubt in this regard, not to mention phenomena like misophonia, which is clearly not a type of synesthesia but for the moment still stubbornly sits on some lists like that of the English-language Wikipedia entry, giving rise to confusion. Mirror touch is a good example of a type which despite being generally accepted as such is a source of discrepancy for some researchers, who would not include it on their own list of types.

For these reasons I don’t believe it is possible to give a specific number and it probably never will be.

Why is it impossible to say how many types of synaesthesia there are?