Marx explaining his materialist method:
"The concrete is concrete because it is the concentration of many determinations, hence unity of the diverse. It appears in the process of thinking, therefore, as a process of concentration, as a result, not as a point of departure, even though it is the point of departure in reality and hence also the point of departure for observation [Anschauung] and conception. Along the first path the full conception was evaporated to yield an abstract determination; along the second, the abstract determinations lead towards a reproduction of the concrete by way of thought. In this way Hegel fell into the illusion of conceiving the real as the product of thought concentrating itself, probing its own depths, and unfolding itself out of itself, by itself, whereas the method of rising from the abstract to the concrete is only the way in which thought appropriates the concrete, reproduces it as the concrete in the mind. But this is by no means the process by which the concrete itself comes into being. For example, the simplest economic category, say e.g. exchange value, presupposes population, moreover a population producing in specific relations; as well as a certain kind of family, or commune, or state, etc. It can never exist other than as an abstract, one-sided relation within an already given, concrete, living whole. As a category, by contrast, exchange value leads an antediluvian existence. Therefore, to the kind of consciousness – and this is characteristic of the philosophical consciousness – for which conceptual thinking is the real human being, and for which the conceptual world as such is thus the only reality, the movement of the categories appears as the real act of production – which only, unfortunately, receives a jolt from the outside – whose product is the world; and – but this is again a tautology – this is correct in so far as the concrete totality is a totality of thoughts, concrete in thought, in fact a product of thinking and comprehending; but not in any way a product of the concept which thinks and generates itself outside or above observation and conception; a product, rather, of the working-up of observation and conception into concepts. The totality as it appears in the head, as a totality of thoughts, is a product of a thinking head, which appropriates the world in the only way it can, a way different from the artistic, religious, practical and mental appropriation of this world. The real subject retains its autonomous existence outside the head just as before; namely as long as the head’s conduct is merely speculative, merely theoretical. Hence, in the theoretical method, too, the subject, society, must always be kept in mind as the presupposition."
Exchange value, far from being metaphysical entity, cannot even exist other than 'an abstract, one-sided relation within an already given, concrete, living whole'. If you flatly contradict Marx, as Phil does, and conceive of exchange value as a thing in itself then it is unsurprising that you end up in strange philosophical waters where metaphysical entities impose themselves on empirical reality. If you remind yourself that the conception of exchange value relies upon the real, living totality of material relations then all the philosophical mysteries vanish.
ETA: It should be added that the above makes it clear that there is no half-way house between materialism and idealism. Either concrete reality comes into existence in the manner that the concrete conception of reality does or it doesn't. Either thought process follows physical process or it doesn't.
