A rogues gallery of historical personages, appearing in rough chronological order of their date (or estimated date) of birth, from (a) to (z), who shaped modern scientific conceptions of sensation and reward. Panel A: (a) Egyptian King Intef II (ca. 2108–2069 BCE) funerary stele. On display at the Metropolitan Museum of Art. (From file “Intef II” by Wikimedia Commons user David Liam Moran, used under a Creative Commons Attribution-Share Alike 1.0 Generic license. File located at http://commons.wikimedia.org/wiki/File:Funerary_stele_of_Intef_II.jpg.) (b) Abram (Abraham) of the Old Testament. Etching, “God’s covenant with Abraham, State 1,” by Wenceslaus Hollar (1607–1677). (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Wenceslas_Hollar_-_God’s_covenant_with_Abraham_(State_1).jpg.) (c) Guan Zhong (Kuan Chung; given name Yíwú), appointed Prime Minister of the Chinese state of Qi in 685 BCE. (From http://history.cultural-china.com/en/47History1499.html and also at http://www.xinfajia.net/english/4680.html. With permission.) (d) Democritus. Image depicted on the reverse of the Greek 10 drachma coin, 1976–2001. (e) Aristippus. Engraving appearing in The History of Philosophy by Thomas Stanley (1655). (f) Aristotle. Bust of Aristotle at the Natural Museum of Rome, Palazzo Altemps, Ludovisi Collection. Marble, Roman copy after a Greek bronze original by Lysippos from 330 BCE. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Aristotle_Altemps_Inv8575.jpg.) (g) Theophrastus. Engraving by John Payne, detail of title page from The herball, or, Generall historie of plantes, by John Gerard (From London: Richard Whitaker, 1633). (h) Epicurus. Bust of Epicurus at the National Museum of Rome, Palazzo Massimo alle Terme. Pentelic marble, Roman copy (first century CE) of a Greek original of the third century BCE. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Epicurus_Massimo_Inv197306.jpg.) (i) Galileo Galilei. Galileo Galilei. Engraving. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Galileo_Galilei_4.jpg.) (j) Thomas Hobbes. Portrait detail (head) of Hobbes, overlaid on the frontispiece of his book, Leviathan, 1651. (Image of body taken from Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Leviathan.jpg.) (k) René Descartes. Detail from oil painting of Descartes, by Frans Hals (1649), National Gallery of Denmark. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Frans_Hals_111_WGA_version.jpg.) (1) John Locke. Lithograph from the Library of Congress, reproduction number LC-USZ62-59655: “by de Fonroug[…]? after H. Gamier. [No date found on item.].” (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Locke-John-LOC.jpg.) Panel B: (m) Jeremy Bentham. “Auto-Icon,” with wax head. Acquired by University College London in 1850. (From file “Jeremy Bentham Auto-Icon.jpg” by Wikimedia Commons user Michael Reeve. File located at http://commons.wikimedia.org/wiki/File:Jeremy_Bentham_Auto-Icon.jpg.) (n) Johannes Müller. Detail of oil painting by Pasquale Baroni. Museo di Anatomia Umana “Luigi Rolando,” Torino, Italy. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Johannes_ Peter_Müller_by_Pasquale_Baroni.jpg.) (o) Gustav Fechner. Photograph. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Gustav_Fechner.jpg.) (p) Alexander Bain. Photograph. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:AlexanderBain001.jpg.) (q) Hermann von Helmholtz. German postage stamp commemorating the 150th birthday of Helmholtz, issued 27 August 1971. (r) Wilhelm Wundt. Photograph, 1902. (Weltrundschau zu Reclams Universum. From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Wilhelm_Wundt.jpg.) (s) Ivan Pavlov. Portrait of Pavlov, 1920. Image taken from the Google-hosted LIFE Photo Archive, available under the filename 6bfe-762c14ea3e8d. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Ivan_Pavlov_LIFE.jpg.) (t) Sigmund Freud. Portrait of Freud, 1920, by Max Halberstadt. Image taken from the Google-hosted LIFE Photo Archive, available under the filename e45a47b1b422cca3. (From Wikimedia Commons at http://commons.wikimedia.org/wiki/File:Sigmund_Freud_LIFE.jpg.) (u) Walter Cannon. (Frontispiece portrait from the article by Mayer, J. Nutr., 1965, reproduced with permission from the American Society for Nutrition.) (v) Edward Thorndike. Image courtesy of the National Library of Medicine. (w) Clark Hull. Anonymous. n.d. [Portrait of Clark L. Hull]. Photograph. (From the collection of Rand B. Evans. From http://vlp.mpiwg-berlin.mpg.de/references?id=lit38366&page=p0001recto. With permission.) (x) Leonard Troland. Image from the Optical Society (OSA), Past Presidents 1922–1923. (Located at http://www.osa.org/about_osa/leadership_and_volunteers/officers/past_presidents.aspx. With permission.) (y) Burrhus Frederic (B.F.) Skinner. Photograph. (From the file “B.F. Skinner at Harvard circa 1950.jpg” by Wikimedia Commons user Silly rabbit. From the file http://commons.wikimedia.org/wiki/File:B.F._Skinner_at_Harvard_circa_1950.jpg.) (z) James Olds. Photograph. (From the article by Thompson, Biographical Memoirs, 1999, with permission of the National Academy of Sciences, U.S.A.)