A term that has largely gone out of fashion, metaphysics was a frequent subject of philosophic inquiry during the Enlightenment. It refers to the study of basic principles of reality and how human beings perceive it. Such facts cannot be observed, as they lie beyond the physical world, and so philosophers attempt to deduce conclusions from logic and available facts. Perhaps the most famous example of metaphysical reasoning is Rene Descartes’s declaration, “I think, therefore I am,” which affirms the reality of human existence through awareness of one’s own cognition.

Natural law is a philosophical concept that tries to identify moral principles that should be binding on all human beings regardless of their particular circumstances. It was closely associated with Catholic philosophers such as St. Thomas Aquinas, who argued that Humanity’s Capacity for Reason was bestowed by God so that people could learn and follow natural law. Rousseau reinterprets natural law as instinct guiding a human life led entirely according to nature, similar to animals that know to follow certain behaviors without any education. Jean-Jacques Rousseau argues that for human beings natural law ordains self-preservation and pity toward sentient beings.