The purpose of the essay "Labour and Exercise" is to discuss how the body stays in good health through both labor and exercise. As the essay was written in 1711, Addison cites labor as the primary way that humans can preserve their health and their bodies in the context of the primary industries of the time.

When Addison writes, "Manufactures, trade, and agriculture naturally employ more than nineteen parts of the species in twenty; and as for those who are not obliged to labour, by the condition in which they are born, they are more miserable than the rest of mankind unless they indulge themselves in that voluntary labour which goes by the name of exercise," the reader can use context to understand the phrase "nineteen parts of the species in twenty."

By "the species," the reader can assume Addison is referring to the human species, since the essay is about the human body and the labour and exercise of it. In the beginning of the sentence, he speaks about the industries that employ "the species," that is, people. In the following sentence, he writes, "as for those who are not obliged by labour," that is, as for the people who don't work. Therefore, the reader can assume that "nineteen parts of the species in twenty" means nineteen out of twenty people, or 95% of the population. The full thought could be paraphrased as follows: 95% of the population works in the manufacturing, trade, and agriculture industries.