An interesting? piano article

[Mark Davidson]

>>There are only three variables that the 
>>pianist's hands can influence to change piano 
>>sound. These are:-
>>	*	 the velocity at which the hammer 
>>strikes the string - the loudness:
>>	*	the time when the hammer strikes the string: and
>>	*	 the duration of sound, 
>>determined by when he releases the key allowing 
>>the damper to fall and stop the string vibrating.
>
>Stephen Birkett writes:
>
>In other words, the author states one side of the 
>famous three-variable question that has raged 
>periodically from time to time for well over a 
>century, as if it was proven fact.
>

What is the "famous three-variable question"?

-Mark