To have a synesthesia it needs two necessary and sufficient conditions: I) the presence of two or more sensorial domains at the same time (senses and/or sensations), that can be real or virtual, distinct and different among the six senses traditionally classified (according to Ullmann's convention that distinguishes the thermic sensation from the touch); II) between the two different sensorial domains must exist a link of synthesis (from analogy to identification) and not of accumulation or parallelism. Adopting this criterion of demarcation, that clearly and strictly isolates the object of the analysis from the confused amount of phenomena, apparently similar, pseudo or parasynesthetic, it is also possible to mark more certainly the historical excursus of the synesthesia to underline its appearance and its most significant stages, even behind the multiform epiphanies under which this phenomenon, proteiform by nature, can disguise itself. That is just what here we tried to delineate indicating the very beginning of two among the three possible synesthetic forms: in re, in intellectu, in verbo