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The Thomist 64 (2000): 271-300 ELEMENTAL VIRTUAL PRESENCE IN ST. THOMAS CHRISTOPHER DECAEN Thomas Aquinas College Santa Paula, California Perhaps the best-known example of how Aristotle's philosophy of nature is thought to have been superseded by the scientific revolution of the seventeenth and eighteenth centuries is the establishment of the atomic theory. The Stagirite's thoroughgoing opposition to the reduction of substances to aggregates of atoms is evident throughout his physical works and to the modern reader it seems that if there is any point on which Aristotle has been proved wrong it is this. It is therefore natural to think that, because they adopted Aristotle's mistake, the medieval philosophers-most notably St. Thomas Aquinas-are similarly outmoded. However, given the considerable revision of our understanding of the existence of atoms underway in contemporary science (particularly in quantum theory) since the beginning of the twentieth century, one is tempted to reassess the degree to which the Aristotelian abhorrence of atomism is truly obsolete.1 1 See Edward MacKinnon, S.J., "ThomismandAtomism," Modern Schoolman 38 (1961): 121-41; William A. Wallace, O.P., "Are the Elementary Particles Real?" in From A Realist Point ofView: Essays on the Philosophy ofScience (2d ed.; Lanham, Md.: University Press ofAmerica, 1983), 171-83. More recently, see Wolfgang Smith, "From Schrodinger's Cat to Thomistic Ontology," The Thomist 63 (1999): 49-63; and The Quantum Enigma: Finding the Hidden Key (Peru, Ill.: Sherwood Sugden and Company, 1995), especially chapters 3 and 4. For another recent but more careful exposition of St. Thomas's doctrine, focused on virtual presence and less concerned with the implications of quantum theory than the others are, see Joseph Bobik, Aquinas on Matter and Form and the Elements: A Translation and Interpretation ofthe De Principiis Naturae and the De Mixtione Elementorum ofSt. Thomas Aquinas (Notre Dame: University of Notre Dame Press, 1998). 271 272 CHRISTOPHER DECAEN While this paper will not offer such a reassessment, it will · provide at least part of what must serve as a necessary foundation for it. In the following I will present a critical exposition of St. Thomas's account of the manner in which elemental substances are present in non-elemental substances, referred to as "mixtures ."2 If this mode of existence, usually referred to as "virtual presence,"3 is not articulated very carefully, it will be not only an obstacle to any attempt at showing the present-day relevance of Aristotelian natural philosophy, but in fact an enigmatic and obscure account of the workings of nature. Virtual presence has received surprisingly little space in the already sparse literature on Thomas's natural philosophy. One 2The words in Aristotle and St. Thomas are µ(1;1i;; and mixtum (or mixtio), respectively. I hesitate in deciding how to translate these words, the main reason being that what modern chemistry refers to as a mixture is something more precise, and probably less substantially united, than that to which Thomas and Aristotle refer. In chemistry mixtures are divided into two categories: heterogeneous and homogeneous. However, the definition of each is primarily operational. A mixture is heterogeneous if it is an aggregate in which the particles are merely juxtaposedand can be mechanically separated, whether by filtration, distillation, or simply by using a pair oftweezers. It is homogeneous (or a solution) if there is such a thorough blending among the parts that the one dissolves in the other and they cannot be mechanically separated. Such mixtures are opposed to chemical compounds, in which there is a much stronger bond between the parts, one that involves the sharing or transferring of electrons on the atomic level (and such are subdivided into ionic and covalent bonds, each of which can also be further subdivided), and which possess propertiesradically different from those of their constituents. An example ofaheterogeneous mixture would be salt and pepper shaken together in a jar; an example of a homogeneous mixture, salt water; and an example of a chemical compound, table salt (sodium chloride). While it is clear that Thomas would not consider a heterogeneous mixture to be a mixtum (in De Mixtione Elementorum, In. 34, he calls such a confusio or a mixtio ad sensum, as...


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