Unknown painter
Unknown painter is anyone who is a painter and whose identity is not known to the public. The identity of the painter is usually known to the painter herself.
Inference and comparative analysis would be used to attribute various paintings of unknown authorship to various unknown painters. However, it has been suggested that there is a small possibility that all paintings whose author is unknown are actually authored by a single individual who is immortal and an accomplished ninja.
This possibility, however, is small. So small, in fact, that mathematicians routinely use the immortal ninja painter probability as an infinitesimal in mathematical proofs.