" This is an absolutely vital point that cannot be overemphasized: value is immaterial but objective. Given Marx’s supposed adherence to a rigorous materialism, this is, on the face of it, a surprising argument, and we have to wrestle a bit with what it means. Value is a social relation, and you cannot actually see, touch or feel social relations directly; yet they have an objective presence. We therefore have to carefully examine this social relation and its expression. "

― David Harvey , A Companion to Marx's Capital