I think the notion that one can't measure abstract labor is based on a
confusion between Marx's categories of "abstract labor" and "socially
necessary labor-time."
The amount of SNLT an individual worker does in a particular span of time
cannot be measured *directly*. But the labor of each worker, right there at
the point of production, has, according to Marx, a "dual character": it is
both concrete and abstract. Abstract labor is labor that lacks specificity;
it is exertion, pure and simple; purely physiological activity. It is
precisely because it is abstracted from all specificity that all abstract
labor can be measured in the same abstract units -- clock-time -- and added
together, etc.
This is not possible with concrete labor. An hour of weaving-labor is not an
hour of labor; an hour of tailoring-labor is not an hour of labor. Only an
hour of (abstract) labor is an hour of labor.
Hence, it is possible to measure the amount of abstract labor by the clock.
It is not possible *directly* to say how much of that abstract labor counts as
socially necessary in any particular case.
Andrew Kliman