What are the two main variables in Malthusian theory?

Among the most famous theories of demography is the one associated with the English political economist Thomas Robert Malthus (1766-1834). Malthus’s theory of population growth – outlined in his Essay on Population (1798) – was rather pessimistic. He argued that human populations tend to grow at a much faster rate than the rate at which the means of human subsistence (especially food, but also clothing and other agriculture-based products)