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Fechner - The Founder of Psychophysics:
Many historians of psychology consider Fechner to be the founder of psychophysics and thus the founder of quantitative psychology.22
The publication in 1860 of Fechner's Elemente der Psychophysik, which was translated as Elements of Psychophysics, marks the official birth of the science of psychology.23
It broadly contains the following topics: general considerations on the relation of body and mind, the concept and the task of psychophysics, a preliminary question, concepts concerning sensation and stimulus, the measure of physical activity, the principle of measurement for sensitivity, the principle of measurement for sensation, methods of measuring sensitivity, Weber's law, thresholds, further details on magnitude and relationship of the threshold in various sense domains, the parallel law in relation to Weber's law, and laws of the phenomena of mixture. According to Fechner himself, the first volume of the book contains the foundation of psychics measurement, that is to say, the establishment of its principle and the exposition of the methods, laws, and data which belong to its empirical proof. The second volume of the book developed the functions of psychic measurement together with their implications, which shift from the outer to the inner sphere in the mind-body relation.24
Fechner, the major proponent of psychophysics, also attempted to explore more fully the relationships between sensations and perception.25 According to him, sensation depends on stimulation; a stronger sensation depends on a stronger stimulus; the stimulus, however, causes sensation only via the intermediate action of some internal process of the body. To the extent that lawful relationships between sensation and stimulus can be found, they must include lawful relationships between the stimulus and its inner physical activity, which obey the same general laws of interaction of bodily processes and thereby give us a basis for drawing general conclusions about the nature of this inner activity.26 By psychophysics, Fechner meant a theory which was new insofar as its formulation and treatment are concerned; in short, it was a theory of the relation of body and mind. As an exact science, psychophysics, like physics, must rest on experience and the mathematical connection of those empirical facts that demand a measure of what is experienced or, when such measure is not available, a search for it. Since the measure of physical magnitudes is already known, the first and main task of this work is to establish the as-yet-nonexistent measure of psychic magnitudes; the second is to take up the applications and detailed arguments that developed from it. The determination of a psychic measure is no mere matter of academic or philosophical abstraction but demands a broad empirical basis.27 Psychophysics, already related to psychology and physics by name, must on the one hand be based on psychology and on the other hand give psychology a mathematical foundation. From physics outer psychophysics borrows aids and methodology; inner psychophysics leans more to physiology and anatomy, particularly of the nervous system, with which a certain acquaintance is presupposed.28 The empirical law that forms the foundation of the theory of psychological measurement was advanced long ago by various scholars in diverse areas and was formulated and experimentally proven in relative generality, particularly by Weber, who, in Fechner's opinion really should be called the father of psychophysics. In addition, the mathematical functions that constitute the most general and most important applications of Fechner's principle of measurement were laid down long ago by various mathematicians, physicists, and philosophers, such as Bernoulli (Laplace, Poisson), Euler (Herbart, Drobisch), and Steinheil (Pogson), and are based on special cases that were particularly suited to psychophysics and are reproduced and accepted by other scholars.29 With regard to the manner in which mathematics has been introduced into this work, Fechner wished his writing to be regarded by mathematicians as if written for nonmathematicians, and by nonmathematicians as if written for mathematicians.30 Fechner developed the method of average error (also called the method of adjustment). It consists in having subjects adjust a variable stimulus until they perceive it to be equal to a consistent standard stimulus.31 Fechner's stress on invariance, his operational approach to measurement, and his employment of hypothetical constructs are examples of how close he was to modern modes of dealing with these problems. Elsewhere his work foreshadowed theoretical positions that inevitably strike the current reader as belonging to contemporary psychological theory. In volume 1, he took the first steps toward a quantitative psychology and staked his claim to his being the "father of psychophysics" and the pioneer experimental psychologist. He brought to scientists and scholars, for the first time, the methods of psychological measurement that still remain the basis of psychophysics, although these procedures are now supplemented by others. These methods were employed to fulfill Fechner's goal of an objective psychology as laid out in 1851 in Zend-Avesta.32 
If Fechner founded experimental psychology, he did it incidentally and involuntarily, and yet it is hard to see how the new psychology could advance as it did without an Element der Psychophysik in 1860.33
Such an empirical and quantitative program was to replace one that had been primarily speculative and philosophical. By 1860, Fechner had brought together a considerable body of experimental work, his own and that of others, to support his thesis.35 Fechner's claim to greatness within psychology does not, however, derive from these psychological conceptions of his, nor even from the formulation of his famous law. The great thing that he accomplished was a new kind of measurement. The critics may debate the question as to what it was that he measured; the fact stands that he conceived, developed, and established new methods of measurement, and that, whatever interpretation may later be made of their products, these methods are essentially the first methods of mental measurement and thus the beginning of quantitative experimental psychology. Moreover, the methods have stood the test of time. They have proven applicable to all sorts of psychological problems and situations that Fechner never dreamed of, and they are all still used with only minor modifications in the greater part of quantitative work in the psychological laboratory today.36 Thus, historians of psychology agree that Fechner with his publication in 1860 of Elements of Psychophysics marks the starting point of experimental psychology.37 He is seen as the founder of psychophysics and experimental psychology. He was the first scientist to give psychology a mathematical foundation, and he took the first steps toward a quantitative psychology. In this part of the study, however, we are going to introduce a new view on the history of psychology, that Ibn al-Haytham with The Book of Optics, and not Fechner, was the founder of psychophysics and experimental psychology. According to Sabra, Ibn al-Haytham's views have been almost totally neglected by historians, and, it seems, for the same reason, historians of philosophy, who are concerned with the history of perception, have usually regarded works on optics as scientific or mathematical and therefore falling outside their domain, whereas historians of science and mathematics have tended to ignore the psychological sections in such works as properly belonging to philosophy. Even historians of optics, who have given attention to Ibn al-Haytham's doctrine of vision, appear for the most part to have assumed that it was feasible to elucidate the account presented in Book I of The Book of Optics without exploring the subsequent account on perception, as if the two could be meaningfully divorced from one another.38 The area into which psychologists study the link between variation in physical dimension and psychological dimension is called psychophysics, and the methodology used to describe this link is termed experimental psychology. This description of psychophysics is found in Books II and III of The Book of Optics by Ibn al-Haytham. My aim is simply to present The Book of Optics as I understand it, therein justifying my belief that it laid the foundation of psychophysics and experimental psychology.
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