All our inventions have endowed material forces with intellectual life, and degraded human life into a material force.

There are no bigger donkeys than these workers.... Look at our 'craftsmen'; Sad that world history should be be made with such people.

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)

Natural science will in time incorporate into itself the science of man, just as the science of man will incorporate into itself natural science: there will be one science.

When commercial capital occupies a position of unquestioned ascendancy, it everywhere constitutes a system of plunder.

Under no pretext should arms and ammunition be surrendered; any attempt to disarm the workers must be frustrated, by force if necessary.

Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.