In the first book that Marx and Engels wrote together, The Holy Family, there is a passage about the Jacobin leader Saint-Just, who was famous not only for his ruthless conduct of the Terror, but for the intensity with which he urged ideals of civic virtue drawn from the ancient world: his demand, as he expressed it, that revolutionary men should be Romans.

‘There is something tragic’, Marx and Engels wrote

My aim is to start out from Saint-Just's illusion, and by asking what made it an illusion, raise a question about the interpretation of ethical and political ideas, such as freedom, in different times and circumstances. This will lead us to consider a more extreme situation in which we have to interpret the life of other groups of human beings without sharing a history with them. That will lead us, finally, to some thoughts about moral philosophy and what it can do.

The idea which Marx and Engels put in that way, in terms of Saint-Just's illusion, had been expressed before, most notably by Benjamin Constant in his famous lecture twenty-five years earlier on ancient and modern liberty.