The Human Body as an Instrumental Cause of Actions in the Writings of Thomas Aquinas

Abstract

The Human Body as an Instrumental Cause of Actions in the Writings of Thomas Aquinas John Rziha Flowing from its Greek and Judeo-Christian roots, the idea that humans are composed of a body and a soul has been a mainstay of Western thought. Yet, how the soul and body are related has been the subject of much debate going back at least as far as Plato and Aristotle. In modern times, the relation between the body and soul is widely disputed, with many people completely rejecting the existence of the soul (known in philosophical circles as materialists or physicalists).1 Although the materialist rejection of the soul is often based on deep philosophical assumptions, some contemporary materialists believe that the findings of modern neuroscience disprove the existence of the soul (or at least make speaking about a soul irrelevant).2 They note that thoughts, feelings, and even love [End Page 1203] can be measured though physiological changes in the body. For example, electroencephalography (EEG) can measure brain waves, and by studying this and other forms of biofeedback, scientists can tell a lot about what a person is thinking and feeling. From these scientific advances a materialist might argue that modern neurobiology, endocrinology, and psychology can explain all the actions formally attributed to the soul. Furthermore, these actions can be mapped to different functions and parts of the brain and body with the conclusion that there is no integrated self (or soul) underlying actions such as thought or free choice.3 These scientific discoveries might be a challenge to someone who believes the body and soul to be two distinct substances (similar to Descartes).4 However, a person with a proper hylomorphic understanding of the relation between the body and soul, should not be troubled by these scientific findings. In fact, not only do these scientific findings not disprove the existence of the soul (or make it irrelevant), those with a proper understanding of the relation between the body and the soul would expect science to discover that the body causes human actions. This article will examine Thomas Aquinas's understanding of the body as the instrumental cause of actions in order to help explain both the role of the soul and the body within human actions. This essay is not meant to convince the materialist that a soul exists, but it should show that it is quite rational to both believe the findings of modern neuroscience and hold that the human soul [End Page 1204] exists. In order to make these points I will explain four essential aspects of Thomas's understanding of primary and instrumental causality and directly apply these concepts to the particular case of the relation between the body and the soul. This essay is only meant to introduce these ideas and hopefully stimulate other philosophers and theologians to work on this topic to apply it more thoroughly to contemporary debates. As is well-documented, Thomas speaks of the soul as the form of the body. This is the hylopmorphic view of the relation between the body and soul.5 What is less well-documented is that Thomas further explains the interactions between the body and soul by speaking of the soul as the primary cause and the body as the instrumental cause. In his commentary on De anima, Thomas comments on the following words of Aristotle: "For all natural bodies are instruments of the soul: whether of animals or of plants, they exist as for the sake of the soul." In his commentary on the passage Thomas states: "For it is evident that all such bodies are, as it were, instruments of the 'soul'—not only of animals' souls but of the plant soul as well."6 The language of the body as an instrumental cause comes from Aristotle, and Thomas will continue to use it throughout his corpus.7 Although the term instrumental cause can also refer to animal and plant bodies, the present article will be solely concerned with human bodies and souls. To better understand what Thomas means when he says the body is the instrument of the soul, I will give some general background information on the Thomistic concepts of...