Final answer:
Thomas Hobbes is the author who believed that life without centralized, formal leadership would be nasty, brutish, and short, and advocated for a social contract with a monarch to ensure social stability and order.
Step-by-step explanation:
The author who suggested that life without centralized, formal leadership, such as a monarch, would be nasty, brutish, and short is Thomas Hobbes. Hobbes argued that in a state of nature, humans would live in constant fear and danger of violent death, leading to a life that is solitary, poor, nasty, brutish, and short. This bleak outlook on human nature led him to advocate for a social contract where individuals give up some of their freedoms and submit to the authority of an absolute sovereign, typically a monarch, in exchange for peace and social order. According to Hobbes, this establishment of a political community and a centralized authority was essential for maintaining security, enforcing laws, and avoiding the chaos of the state of nature.
Hobbes's philosophy has been influential in shaping the concept of modern governance and the social contract theory, which plays a foundational role in contemporary political philosophy. In 'Leviathan' (1651), Hobbes laid out his views on how a society should be structured to escape the state of nature, favoring monarchy as the best form of government to maintain harmony and prevent the perceived natural state of war among individuals.