Page "Conservation of mass" Paragraph 16

The principle of matter conservation may be considered as an approximate physical law that is true only in the classical sense, without consideration of special relativity and quantum mechanics.

Another difficulty with the idea of conservation of " matter " is that " matter " is not a well-defined word scientifically, and when particles that are considered to be " matter " ( such as electrons and positrons ) are annihilated to make photons ( which are often not considered matter ) then conservation of matter does not take place, even in isolated systems.