61905
During the 19th century , Karl Marx and Friedrich Engels extended the concept of materialism to elaborate a " materialist conception of history " centered on the roughly empirical world of human activity ( practice , including labor ) and the institutions created , reproduced or destroyed by that activity .
 Karl Marx, Friedrich Engels
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: media, plant, vehicle, astronomical object, time, disease, animal, location, person, organization, mythical figure, food, biology, geographical phenomenon, instrument, event 
Text: {0} 
Answer:
 Karl Marx, Friedrich Engels