Once the inner connection is grasped, all theoretical belief in the permanent necessity of existing conditions collapses before their collapse in practice -- Letter to Ludwig Kugelmann (July 11, 1868)

It is not the consciousness of men that determines their being, but, on the contrary, their social being that determines their consciousness.

The philosophers have only interpreted the world, in various ways: the point, however, is to change it.

The history of all previous societies has been the history of class struggles.

The working class is revolutionary or it is nothing.

Philosophy is to science as masturbation is to sex.

The Communists have no need to introduce free love; it has existed almost from time immemorial.