[C]ommon sense... once had been the ["sense"] by which all other senses, with their intimately private sensations, were fitted into the common world, just as vision fitted man into the visible world[. But] now [it has become] an inner faculty without any world relationship. This sense now [is] called common merely because it happened to be common to all. What men [sic] now have in common is not the world but the structure of their minds, and this they cannot have in common, strictly speaking; their faculty of reasoning can only happen to be the same in everybody. The fact that, given the problem of two plus two we all will come out with the same answer, four, is henceforth the very model of common-sense reasoning.
Note that in denying that humans can have the structure of their minds identically in common, Arendt is not denying that there is obvious salient structural overlap in that structure (in fact, she takes this for granted by the paragraph's end), she is denying instead that salience is restricted to the ways in which they do overlap. Philip Rieff's encomium to Freud, that he "democratized genius by giving everybody a creative unconscious," is very much in point here. That we might indeed be neurocomputationally identical in our shared recourse to instrumental rationality provides little reason to imagine we are comparably identical in our capacity to make meaning, express value, divert literal into figurative language, unpredictably interrupt custom and calculation and so "change the subject" in the deepest sense and so on. To deny that "strictly speaking," we actually "cannot have in common" the "structure of [our] minds" is actually to a certain extent little more than the facile materialist admission that we think with different brains even when we think with them similarly, but also that the material structure of these different brains surely materially attest to the different memories, customs, dispositions they incarnate. Be that as it may, Arendt is freighting this distinction of instrumental as against deliberative conceptions of common sense with extraordinary significance. She continues:
Reason, in Descartes no less than in Hobbes, becomes "reckoning with consequences," the faculty of deducing and concluding, that is, of a process which man at any moment can let loose in himself. The mind of this man -- to remain in the sphere of mathematics -- no longer looks upon "two-plus-two-are-four" as an equation in which two side balance in a self-evident harmony, but understands the equation as the expression of a process in which two and two become four in order to generate further processes of addition which eventually will lead into the infinite.
Here, I believe, we find the gesture of superlativity in perhaps an unexpected place, perhaps a foundational place, in which an initial reduction or impoverishment of reason into instrumentality is compensated by a promissory amplification of instrumentality, means without end or ends, functionally substituting force for freedom.
This faculty [ie, instrumental calculation] the modern age calls common-sense reasoning; it is the playing of the mind with itself, which comes to pass when the mind is shut off from all reality and "senses" only itself.
It is crucial to grasp that the "reality" from which this instrumentalized mind is cut off is the substantial reality of the public sphere, the world in common made and sustained by peers acting in concert. Arendt is not literally mistaking instrumentalization as a kind of comatose state, although she would likely point out that both states amount to the radical objectification of a subject no longer able legibly to act in the world on her own. Arendt is not denying that objects of calculation have an alterity that can frustrate our ends and confound our expectations, but proposing that they must first be constituted as objects, subsumed within our conventions, to be legible as frustrating or confounding in salient ways in the first place. Under the mode of instrumentalization a thing to be known must first be made by us or made-knowable by us, we can only trust what we make. This is an argument she elaborates at great length prior to the passage I have quoted here, in an extended reading of the Cartesian Doubt. As always, there is much more to say here than I have time to say it where Arendt is concerned. For now, I will treat the next couple of sentences continuing from the passage above as a conclusion of this particular discussion:
Whatever difference there may be [between intelligent individuals, once their intelligence is reduced to instrumentality] is a difference of mental power, which can be tested and measured like horsepower. Here the old definition of man as an animal rationale acquires a terrible precision: deprived of the sense through which man's five animal senses are fitted into a world common to all men, human beings are indeed no more than animals who are able to reason, "to reckon with consequences."
What intrigues me about this last comment is that while it seems to decry the reduction of human beings to the status of animals (a reduction that has never much disturbed me, since I am quite happy to concede nonhuman animals their share of dignity and a stake in the collaboration of peers in sharing and making a world worth living in), it seems to mark more emphatically in fact the further reduction of human animals to mere mechanisms, an altogether more troubling move it seems to me, and one prone to all sorts of mischief.