The Principles of Communism by Friedrich Engels (Karl Marx's buddy)

2- What is the proletariat?

The proletariat is that class in society which lives entirely from the sale of its labor and does not draw profit from any kind of capital; whose weal and woe, whose life and death, whose sole existence depends on the demand for labor – hence, on the changing state of business, on the vagaries of unbridled competition. The proletariat, or the class of proletarians, is, in a word, the working class of the 19th century.

vagaries- unexpected changes in a situation