There are many different answers to the question of what Marx and Spinoza have in common, theories of ideology, materialism, naturalism, and so on, to name a few that have been discussed on this blog. To this list Margherita Pascucci adds that perhaps what Spinoza and Marx have in common is the common itself. This is is claim put forward in Potentia of Poverty: Marx Reads Spinoza (part of the Historical Materialism series, currently it is only out as a hardcover, but it will be out from Haymarket in the Spring).
In making such a claim Pascucci focuses on the intersection of not just Marx and Spinoza, but the way in which they both assert in different ways, the primacy of the epistemology and ontology of relation. As Pascucci writes,
"The commodity in Marx and the common notion in Spinoza are both defined through an other. This 'other' which defines them is the common among two or more things. In the case of the commodity, this common has a character of abstraction--it disappears at a certain point; in the case of common notions, this common is something material, that which, common to a body and other bodies, brings the trace of relation and allows for knowledge."
As they used to say in graduate school, lets unpack this claim. First, we have the common notion in Spinoza, the second kind of knowledge, beyond the imagination. As Spinoza writes in Proposition 37 and 38 of Part Two of the Ethics:
P37. What is common to all things, and is equally in the part and in the whole, does not constitute he essence of any singular thing.
P38 Those things which are common to all, and which are equally in the part and the whole, can only be conceived adequately.
Common notions are understood in terms of both their genesis and their logic. In terms of their genesis let us begin with the simple and most basic encounter, walking around in the house in the dark I bang my shin against something, I do not know what. This is an encounter marked by pain and confusion, by the affects of sadness and hate. Those affects give shape to what could be called the inadequate ideas in which how something affects me and what something is are confusedly muddled in my scream of "ow, shit! what the fuck?" In that encounter there is still something in common, something that can be conceived adequately, I know something about my body, its materiality, and about whatever I ran into in the dark. I know that it is matter too, it has density and hardness. This commonality is incredibly general, but it is the basis for the construction of other common notions.
Later, in Proposition 40, Spinoza contrasts common notions, which do not define the essence of any singular things, with the universal. The universal is attempt to define the essence of a singular thing, to understand what quality defines humanity, as rational or political animal, or even featherless biped. However, the problem with this particular essence is precisely the variability of particulars. As Spinoza writes,
"But it should be noted that these notions are not formed by all in same way, but very from one to another, in accordance with what the body has been more often affected by, and what the mind imagines or recollects more easily. For example, those who have more often regarded men's stature with wonder will understand by the word man an animal of erect stature. But those who been accustomed to consider something else will form another common image of men--for example that man is an animal capable of laughter, or a featherless biped, or a rational animal."
In contrast with a universal burdened with an often unstated particularity we have the common as that which is common to all and particular to none.
Okay, what does this has to do with the commodity? Here one only has to think of Part One of Capital. Value can only be expressed in relation. This is the point of all those formulations about linen and coats. As Marx writes, "The value of linen as a congealed mass of human labour can be expressed only as an 'objectivity' [Geganständlichkeit], a thing which is materially different from linen itself and yet common to the linen and other commodities."