In his introduction to The Sense of Mystery1 , Fr. Reginald Garrigou-Lagrange, O.P., notes the following:
...nothing is empirically clearer than is the difference that separates two animal species, such as the eagle and the lion, or two plant species, such as the oak and the fir. However, how are we to define these species except in a manner that is descriptive or empirical? How are we to render them intelligible? Only their generic characteristics—"corporeal substance," "living," "endowed or not endowed with sensation"— come to the level of intelligibility. However, the specific difference of the oak, the fir, the eagle or the lion remains hidden to us. It is impossible to have a distinct intellectual knowledge of them from which we could deduce from their natures the properties of these natures as we do in the case of, for example, the circle or the triangle. And why is this the case? It is so because their specific (i.e., substantial) form remains, so to speak, buried and immersed in matter. And, thus, the human idea of the eagle or the lion is like a mountain with a clear summit but with a base that remains in the shadows. The eagle and the lion are clearly intelligible for us inasmuch as they are beings, inasmuch as they are substances, corporeal substances endowed with life and with sensation. However, that which formally constitutes the eagle as an eagle or the lion as a lion remains very obscure for our intellect, and we hardly surpass a descriptive and empirical definition. St. Thomas also noted this fact often, namely that the specific differences of sensible beings are often unnamed. There is a dearth of names because their is a dearth of ideas2 .
As he notes, animal and plant species are not as easily susceptible to definition as a mathematical object, such as a triangle or an integer. Species are not easily definable, in part because traits that make them distinct from other species are variable. Furthermore, one often finds it necessary to tighten the definition of an existing species to distinguish it from a newly discovered one.
The original French edition of this study was written in 1934, when there were many ideas in biology and chemistry that were beginning to produce a powerful synergy. Gregor Mendel's mid-19th century theory of biological inheritance received a shot in the arm with the Boveri-Sutton chromosome theory thanks to the work of Thomas Hunt Morgan, followed by that of Ronald Fisher. Indeed, it was soon after Fr. Garrigou-Lagrange's book was published that the field of molecular biology was recognized as a separate discipline in the scientific community3 .
The science of genetics is augmented by the science of taxonomy to classify individual biological species into related groups in a taxonomic hierarchy and to explain how the traits of distinct species arise from their underlying genetics at the DNA level and the role played by these genes in protein synthesis. The combination of comparing genetic structures to macro-level differences in traits offers the best hope, to date, of making sense out of the differences in species that Fr. Garrigou-Lagrange recognized suffered from a complexity that could not be reduced to the simplicity of geometry or number theory. Nevertheless, it is possible to recognize an elaborate taxonomy in species differentiation just as we can see an elaborate taxonomy of disciplines in the sciences and mathematics.
Furthermore, one can begin to see the analogy between the conceptual hierarchies of Aristotle and Aquinas and the species hierarchies that occur naturally through genetic variation encoded in DNA. Indeed, we may some day see the impact of specific genetic differences that facilitate the study of specific scientific or mathematical, or other, disciplines, something never envisioned by the classical thinkers.
- 1Translation by Matthew K. Minerd taken from the 2017 edition issued by Emmaus Academic. The quote is found on p. xlii in the "Author's Introduction"
- 2Fr. Garrigou Lagrange bemoans the fact that there is no simple definition of "eagle", "lion" or of other species, although he goes on to remark that man's intellectuality separates him from all other animal species, and therefore can be used to define him as distinct. We might be able to say the same thing about our consciousness, our conscience or our free will.
- 3See Mendelian inheritance and History of molecular biology.