This work is suppressed since 2010 and led to established scientists to be banned on the arXiv. We thank viXra for making critical work like this still available to relatively uncorrupted parts of the scientific community. We define a mechanical analog to the electrical basic circuit element M = dφ/dQ, the ideal mechanical memristance M = dp/dx, p being momentum. A never before described mechanical memory resistor M(x) is independent of velocity v and has a pinched hysteretic loop that collapses at high frequency in the v versus p plot: a perfect memristor. However, its memristance does not crucially involve inert mass, and the mechanical system helps clarifying that memristor devices hypothesized on grounds of physical symmetries require more. The missing mechanical perfect memristor needs to be crucially mass-involving (MI) precisely like the 1971 implied memristor device needs magnetism. Discussing novel MI memristive systems clarifies why such perfect MI memristors and EM memristors have not been discovered and may be impossible.