So much of our lives has been subsumed by drugs and medicine—do we really need another ‘pill’ to add to the mix? This brief epilogue argues that the answer is, actually, no. We need fewer, but better drugs—drugs with less severe side-effects, and more power to genuinely improve our well-being. The potential of MDMA and some psychedelics to replace a range of harmful medications is discussed, with a renewed call for high-quality research into this possibility as applied to relationships.

This chapter introduces the central ideas in Darwin’s Expression, poses the main interpretive questions that scholars have raised, and outlines my answers to those questions. Why does Darwin analyze expressions in terms of heritable habits, recalling Lamarck’s debunked theory of evolution, when his own theory of natural selection provides a superior alternative? My answer is that Darwin embraces Hartley’s associationist theory of mind, which posits habit as the basis of thought. I claim that multiple puzzling features of Expression are resolved once we view Darwin as an associationist philosopher.

This chapter examines Darwin’s taxonomy of emotions. I show that nineteenth-century associationists were divided on whether emotion categories exist in nature or are conventional. Herbert Spencer argues that emotion categories exist in nature, anticipating modern basic emotions theory. Thomas Brown argues that emotion categories are conventional, anticipating modern psychological constructionism. I show that Darwin sides with Brown and regards emotion categories as conventional. This finding is surprising, since Darwin is often viewed as a precursor to modern basic emotions theory, which adopts the opposite view.

This chapter argues that Darwin’s philosophical theory of emotion has been forgotten due to paradigm shifts in biology, psychology, and philosophy. These shifts have caused researchers to neglect associationist theories of emotions, including Darwin’s contributions to this school of thought. Having explained why Darwin’s philosophy was forgotten, I conclude by explaining why it should be remembered, given its relevance for contemporary emotions research.

This chapter traces the development of Darwin’s theory of emotion and expression from 1838 to 1872, emphasizing his many engagements with associationist philosophers. I demonstrate that all three of Darwin’s principles of expression are derived from the works of associationist philosophers, especially David Hartley, Erasmus Darwin, Herbert Spencer, and Alexander Bain.

The Introduction presents the main theses of this book: that Charles Darwin developed a philosophical theory of emotion, inspired by his reading of several associationist philosophers; that Darwin denied that emotional expressions evolved as social signals, designed to reveal emotions to others; and that Darwin’s theory of emotion has more in common with modern constructionist theories than with modern basic emotions theories, which often claim Darwin as their inspiration.

This chapter surveys associationist theories of emotion leading up to Darwin’s Expression. These theories analyze emotions as sequences of thoughts, feelings, and actions, linked together by principles of association. Thomas Hobbes contributes to this tradition the idea that emotions can be analyzed as “trains of thoughts.” John Locke contributes the idea that these trains are connected by the “association of ideas.” David Hume contributes the idea that association can occur via contiguity, resemblance, or cause and effect. David Hartley puts these ideas together to present the first full-fledged associationist theory of mind and emotion. Harley’s ideas are developed further by Joseph Priestley, Erasmus Darwin (Charles’s grandfather), Thomas Brown, James Mill, Alexander Bain, and Herbert Spencer, among others. This tradition in the philosophy of emotion has never before been described or analyzed.

This chapter examines Darwin’s analysis of emotional expression. It is widely accepted that Darwin wrote Expression to refute Sir Charles Bell’s theory that God created humans with special muscles to express their emotions. However, scholars have overlooked the fact that Bell developed his theory to refute Erasmus Darwin’s associationist analysis of emotional expression, inspired by David Hartley, and that Charles Darwin defends his grandfather’s analysis against Bell’s objections. I demonstrate that Charles’s defense of Erasmus’s associationist theory, which denies that expressions occur for the sake of communicating emotions, explains Charles’s puzzling reluctance to claim that expressions evolved to serve as signals in communication.

The obligation to support space exploration can be defended in at least three ways: (1) the ‘argument from resources,’ that space exploration is useful for amplifying our available resources; (2) the ‘argument from asteroids,’ that space exploration is necessary for protecting the environment and its inhabitants from extraterrestrial threats such as meteorite impacts; and (3) the ‘argument from solar burnout,’ that we are obligated to pursue interstellar colonization in order to ensure long-term human survival. However, even if we accept all three propositions, that space exploration will give us access to asteroidal and other resources; will allow us to defend ourselves against meteorites (by intercepting or destroying them); and finally that interstellar colonization might be useful in saving us from solar burnout, it does not follow that we have an obligation to do any of those things. What follows is that we have reasons to take those actions as practical measures that will bring about the ends in question. But no obligation per se arises from the fact that those measures will be helpful in attaining those ends.