John Locke 's An Essay Concerning Human Understanding and Thomas Malthus 's An Essay on the Principle of Population are long essays . Almost all modern essays are written in prose , but works in verse have been dubbed essays ( e.g. Alexander Pope 's An Essay on Criticism and An Essay on Man ) . While brevity usually defines an essay , voluminous works like John Locke 's An Essay Concerning Human Understanding and Thomas Malthus 's An Essay on the Principle of Population provide counterexamples .