Nahezu jede naturwissenschaftliche Epoche befasste sich mit der Frage, ob oder unter welchen Umstanden der Mensch als willensfrei und mental verursachend beschrieben werden kann. Was zwischen Descartes, den Influxionisten, Husserl und der modernen Neurophysiologie zu verhandeln gewesen ware, will somit als Leit- und Grundmotiv der Fragen nach mentaler Verursachung und Willensfreiheit immer wieder in die Welt treten. Neu sind heute aber das Vokabular und die Daten, die der Gegenwartsdebatte ihre Gestalt verleihen. In diesem Zusammenhang wird insbesondere Libets Experiment zur Willkurmotorik (...) als empirisch eingelautetes "Ende der mentalen Verursachung" ins Treffen gefuhrt. In diesem Buch werden diese Befunde diskutiert und allgemeiner die Rolle, die das Bereitschaftspotential in Hinblick auf intentionale Handlungen einnimmt. Diese Diskussion zeigt, dass die freiheitsskeptische Deutung der Befunde von Libet (und einiger Nachfolgestudien) einer Analyse der Gesamtbefunde nicht standhalten kann. (shrink)

Benjamin Libet's celebrated experiments concerning freedom elicited numerous attempts of _philosophical_ repudiation. Ten years ago, however, Judy Trevena and Jeff Miller published a _technical_ objection; they claim to have detected a,,smearing artifact" in Libet's calculations. This rests on a misunderstanding of Libet's methodology. In my reconstruction of Libet's argument, he draws an abductive inference to the best explanation. Now, Trevena's and Miller's objection does indeed lead to alternative explanations of Libet's measurements. These alternatives are _ad hoc_ and extremely improbable. They (...) constitute worse explanations than the explanation offered by Libet. _German_ Benjamin Libets bahnbrechende Experimente zur Willensfreiheit aus den Achtziger Jahren des vorigen Jahrhunderts gehören längst zu den Klassikern der Experimentalkunst. Aus _philosophischer_ Perspektive sind sie oft kritisiert worden; unabhängig davon ist zu Beginn des neuen Jahrtausends ein _technischer_ Einwand gegen Libet prominent geworden, der von Freiheitsfreunden gerne zitiert wird. Er geht auf Judy Trevena und Jeff Miller zurück und besteht in dem Vorwurf, dass Libet mit seiner Berechnung der _gemittelten_ Bereitschaftspotential-Kurven die wahren Verhältnisse verschmiere und dabei aus mathematischen Gründen den Anstiegs-Zeitpunkt des Bereitschaftspotentials künstlich nach vorne verschiebe – das ist der Vorwurf des sog. Verschmierungsartefakts. Ich zeige, dass der Vorwurf nicht sticht. Er beruht auf einem Missverständnis der Schlussmethode Libets. Laut meiner wissenschaftstheoretischen Rekonstruktion zieht Libet weder deduktive noch induktive Schlüsse aus seinen Einzelbeobachtungen; er schließt abduktiv, d.h. er schließt auf die beste Erklärung seiner Einzelbeobachtungen. Wie eine genaue arithmetische Analyse zeigt, bieten die Alternativ-Erklärungen, unter denen sich das Verschmierungsartefakt bewahrheiten würde, schlechtere Erklärungen als Libets Erklärung – sie sind an den Haaren herbeigezogen und extrem unwahrscheinlich. (shrink)

In lieu of an abstract, here is a brief excerpt of the content:Response to the CommentariesSean A. SpenceIn “Free Will in the Light of Neuropsychiatry,” I have attempted to present an argument from the perspective of materialist neuroscience, pushing the latter to its logical conclusion: that if the human nervous system is consistent in its properties, then the only place for “free will” is in the non-conscious processes which underpin conscious awareness. This argument I have based on two supports: the (...) “spatial” and the “temporal” (Spence, 1995).I have contended that the sense of freedom may be lost from certain actions, thereby implying that “freedom” is a sensory process distinct from action. This might be conceptualized as having a distinct functional distribution within the nervous system (a spatial distinction). I have also argued that conscious awareness of events occurs at a finite delay, the period of “neuronal adequacy,” after those events themselves. The point here is that neurochemical events giving rise to phenomenology are likely to require a period of time (no matter how short) for their genesis. The process is “on line” before we perceive it (a temporal distinction). Thus the “freedom to act,” a motor process, is necessarily non-conscious at the point of initiation.I am grateful to Frith, Libet and Stephens for their comments. In the most part our disagreements are likely to be those of emphasis, perhaps compounded by my uses of the term “free will.”I have no major disagreement with Frith’s comments. I would, however, emphasize that it is likely that the Bereitschaftspotential, or readiness potential, does tell us something regarding intended actions. It is, for example, absent prior to the involuntary movements seen in Tourette’s syndrome (Obeso et al. 1982), and reduced in akinetic patients with Parkinson’s disease (Dick et al. 1989) or medial frontal lobe lesions (Deecke et al. 1987). The latter groups have difficulty initiating intended, internally generated, actions.With regard to the suspicion of dualism in my diagrams, the point is well taken, but I am attempting to describe a functional distinction, the precise coordinates of which remain obscure. It may well be that “consciousness” occupies no fixed location but is an activity temporarily co-extant with that association cortex (or cortices) subserving the sensory modality (or modalities) experienced phenomenologically (see Bottini et al. 1995).I have based much of my “temporal” argument on Libet’s work, and I am grateful for his response. Nevertheless, I am still of the opinion that there is a central inconsistency with his argument that conscious control exerts a veto on acts which is somehow free of material constraints. [Indeed he states that he has “no quarrel with the view that even a fully voluntary act is initiated unconsciously (non-consciously)”.] Whereas I can [End Page 99] not prove that this power of veto does not evade neurochemical processes, or that it somehow acts instantaneously and independently, with no origin in unconscious processing, I can say that I have restricted myself to arguing within a materialist perspective, with the aim of “pushing it” to its extreme. The free will which Libet retains in consciousness is one which is unique, and unconstrained by materialism. It is dualism in disguise!Stephens implies that there is a non-argument regarding consciousness acting as an initiator of events. The suggestion is that no one would argue this. Yet this is Libet’s argument. It cannot be disproved, but it is clearly (internally) inconsistent with Libet’s own impressive body of work.As for the role of consciousness in action, I have, I think, invoked a position similar to that adopted by Stephens; that consciousness is more a sensory state, allowing awareness of actions, and their consequences, and perhaps thereby facilitating the function of non-conscious initiators. It is clearly necessary for the performance of the “higher” acts which we equate with human nature, but it is not sufficient. (Indeed many of the most creative insights have “emerged” into consciousness [see Boden, 1992].) The man with a paralyzed arm may wish for his arm to move but to no avail. Over time function may slowly reappear. But if his lesion is such that he is unaware of his impairment (anosognosia), then his prognosis may... (shrink)