[Marxism] If you want to identify the ruling class, you have to decide what is a social class.

[S. Artesian]

More on this:

Marx uses the term constant to describe the means of production, the 
machinery, and the materials used in production, not because there value was 
always the same no matter the period of production, but because their value 
was preserved in the commodity.

Variable capital was variable not based on the period of production, but on 
the basis that it adds fresh value to the process, AND because the ratio 
within that labor process of/between  two components--between the equivalent 
value of its own reproduction and the surplus value, varies with the length 
of the working day, the technical development of production, etc.