“…It is the entitative character, then, of the 95 % of classical multiple sclerosis that has to be explained by any acceptable hypothesis of causation. Far from reproducing this entitative character in 95 % of the lesions they cause, the agencies so far blamed for multiple sclerosis-such as spirochaetes and neurotropic viruses (e.g., Bailey et al, 1949), thrombosis (Putnam, 1933(Putnam, , 1935(Putnam, , 1937Putnam and Alexander, 1939), or lead-poisoning (Campbell et al, 1950)-can only very rarely produce any lesions which even mimic multiple sclerosis.…”