Final answer:
Friedrich Engels, a key figure in the development of Marxist theory, published significant works like 'The Condition of the Working Class in England' based on his observations.
Explanation:
Friedrich Engels, a German philosopher, social scientist, journalist, and businessman, is well-known for founding Marxist theory alongside Karl Marx. In 1845, Engels published The Condition of the Working Class in England, which was based on his personal observations and research in Manchester.
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