> if anyone else is still reading, what do you suppose Marx meant by
> this sentence in the quote below? It's metaphorical, not literal but I'm
> not sure what "ad hominem" is standing for here:
Probably Ted already answered this, in clearer and more detailed terms. But, quickly, my understanding is that this notion of ad hominem goes back to Aristotle's Metaphysics and Organon.
In Aristotle's Metaphysics, the most fundamental tenets used in logical reasoning (and demonstration) can never be "fully" proved. They can only be claimed ad hominem, i.e. by direct appeal to the character and circumstances of the people involved in the argument. That's what makes them "axioms." Some people have viewed Godel's incompleteness theorems as the modern version of this Aristotelian insight.
Most likely, the use of ad hominem in Marx is also connected with Hegel's critique of Kant. Hegel doesn't really follow Kant (or Aristotle) into an argument on arguments. His Logic entirely shifts the ground. As it unfolds into the actual world, the Idea is self-contradictory, which accounts for its movement: human history is its supreme manifestation. Everything actual in history "possesses and expresses the Idea" -- wrote Hegel in the Notion. What is actual in history has been actualized by people, individuals and -- above all -- world- or universal-historical individuals (e.g. Alexander, Napoleon, etc.). In this sense, the Idea *is* ad hominem.
Marx used the term ad hominem in various contexts, but always in this sense. I can't search it now, but I remember a letter he wrote to Engels just prior to the publication of Capital, volume 1, where he said (quoting by heart here) "In my treatment of capitalist production, I have perused the English Blue Books, depicting the conditions of working people in England (and Ireland) in the greatest detail. As you will note, this is entirely ad hominem." Etc.
The idea implicit here is that the inherent subjective logic of a theory is not immaterial or irrelevant, but it can never be dealt separately from its connection (at the rawest level) with the concrete needs and circumstances of concrete, historically-determined individuals.