The fundamental contribution of St. Thomas Aquinas to the historical advance of philosophy and through philosophy to the advance of Christian theology --- in my opinion but certainly not of all his admirers--- was to accept fully an Aristotelian epistemology freed of the relics of Platonism. While this is by no means Aquinas' only point of originality, it is what made possible his many advances over the thought of Aristotle since these are all based on Aristotle's epistemology and scientific methodology as found in the Stagirite's Posterior Analytics. An especially important consequence of Aquinas' Aristotelianism was that it enabled him to present both a philosophical and hence a theological anthropology free of the Platonic dualism that identified the human person with the soul rather than the ensouled body. Thus Aquinas was enabled by Aristotle himself to surpass Aristotle by developing on the foundation of Aristotle's thought a much clearer account of the spirituality of the human intellect and the human soul than the Greek had been able to achieve.

Essential to this anti-Platonic anthropology of Aquinas is his analysis of the external and internal senses as faculties of the spiritual soul informing bodily organs as this is most fully and maturely formulated in the Summa Theologiae I, q. 78, aa. 3 and 4. There St. Thomas distinguishes the five exterior and four interior senses and shows why they are needed and how they are interrelated in the service of the human intelligence. Father George P. Klubertanz, S.J., in an excellent and very thorough historical and analytic study, The Discursive Power Sources and Doctrine of the "Vis Cogitativa" According to St. Thomas Aquinas ⁽¹⁾ showed that Aristotle himself never clearly distinguished between the exterior and interior senses or ennumerated all the four interior senses or made clear that they are distinct powers. In particular Aristotle only implicitly wrote of the vis cogitativa as the interior sense that in animals is instinct and in human beings the highest of the interior senses because it mediates been the other interior senses and the spiritual human intelligence. It was the Arabian Muslim Avicenna who first began this post-Aristotelian development to which other latter writers contributed but without sufficient consistency or clarity. It was Aquinas who gradually developed this analysis more systematically in his writings and finally expressed it fully and precisely in the articles of the Summa Theologiae just cited.

A.M. Festugiere, O. P., ⁽²⁾ in an earlier and important essay that is not, however, cited by Klubertanz, had shown that Aquinas' ordering of the biological works of Aristotle in his commentary on the latter's De Sensu et Sensato where he places the De Anima first ⁽³⁾ because it deals with the most general concept of life is not that of Aristotle himself. Instead Aristotle, closely followed by Aquinas' teacher St. Albert the Great, ⁽⁴⁾ places the De Anima after the De Partibus Animalium and that after the Meteorologica in which Aristotle studied the chemical composition of mixed terrestrial bodies. Aquinas, on the contrary, places the De Anima first, then second a De Animalibus et Plantis, and thirdthe Parva Naturalia that also follow (but not immediately) the De Anima in Aristotle's own order. Festugiere shows that William of Moerbeke's translation of the De Partibus antedates by several years Aquinas' commentaries on the De Anima and the De Sensu et Sensato and concludes that he must have known Aristotle's plan but deliberately chose not to follow it.

Nevertheless, in my opinion, to explain this departure of Aquinas from Aristotle's ordering of biological investigation two other factors must be taken into account. The first is that the work which Aquinas names in the De Sensu et Sensato as the De Animalibus et Plantis was a conflation of two genuine Aristotelian works, the De Historia Animalium and the De Partibus Animalium together with a spurious work De Plantis ⁽⁵⁾. Second, St. Thomas' ordering can be defended by noting that while Aristotle follows the via inventionis or order of investigation, Aquinas follows the via doctrinae or order of demonstration. I would suggest that perhaps the reason he preferred the via doctrinae was that this conflation of the Historia and the De Partibus made him unsure of Aristotle's own ordering and that moreover it suited his main interest as a theologian to emphasize human spirituality in distinction from the exclusive materiality of brute animals as Aristotle also had done in the Book III book of the De Anima.

This departure of Aquinas' from Aristotle's investigative order has had, however, in my opinion, an unfortunate consequence for the history of Thomism. It has obscured the consistency with which Aristotle followed his empirical epistemology and scientific methodology in which the via inventionis is emphasized and the via doctrinae serves simply as a synthesis and summary of the results of that empirical investigation. Hence in Aristotle's ordering after the detailed chemistry of the Meteorologica the Historia Animalium comes next because it is a collection of raw data about chemical mixtures that differ from inanimate substances in being alive. ⁽⁶⁾The term Historia (a Latin borrowing from the Greek) is used in this title in the sense of "investigations or researches." He then proceeds in the De Partibus Animalium with close attention to method to classify this data with definitions in terms of the ten categories of changeable being (ens mobile) as developed in the foundational part of natural science provided in his Physics.

Only after this preliminary empirical research and classification of materials that results in a comparative zoology does Aristotle proceed to a general definition of the genus of living things in the De Anima. Aquinas correctly explains that this treatise on the soul as the principle of life precedes the more specific discussion of biological questions that relate to various species of living things contained in the Parva Naturalia. Presumably Aristotle also intended to do the same for plants, and in fact his pupil Theophrastus attempted this in his De Plantis and so did Aquinas' teacher St. Albert the Great in a similar treatise.

A like deficiency can be noted in Aquinas' incomplete De Regimine Principum and his Politics commentary where he does not ground political theory in empirical research as Aristotle had so well done with his study of the actual political constitutions of many Greek cities. ⁽⁷⁾ I have always wondered why we do not find in Aristotle's own works a historia orset of empirical researches as a preface to the Nicomachean Ethics parallel to that on which the Politics is based.

I emphasize this point because it is such lacunae in Aquinas' presentation of Aristotle's whole work that have reinforced the utterly absurd opinion still current among scientists that Aristotle had a "deductive not an inductive" conception of science. In the case of Aquinas this false impression has been intensified by the fact that his anthropology is often presented from his theological expositions whose methodology is necessarily quite different from that of natural science, proceeding as it does from revelation and primarily in view of the spiritual aspect of humanity rather than from what can be learned about humanity from sensory experience.

That Aristotle's De Anima is recognized as a main source of Aquinas' philosophical anthropology is due to the important analysis of it that he wrote as the first of his great Aristotelian commentaries. These were begun in Rome between the end of 1267 and the summer of 1268. Little attention, however, has been paid to Aquinas' commentaries on Aristotle's two Parva Naturalia called De Sensu et Sensato and De Memoria et Reminiscentia that probably followed immediately on the De Anima commentary and were finished in Paris in 1269 before the treatise De Unitate Intellectus in 1270. ⁽⁸⁾

In the Parma edition of Aquinas' works other commentaries on works from the Parva Naturalia, such as De Somniis, are included, but today these are generally judged to be spurious. ⁽⁹⁾

As St. Thomas shows in his Proemium to De Sensu et Sensato, just as the sciences are specifically distinguished among themselves by a differences in the separation from matter, so also within a science, especially within natural science, the parts are divided according to separation or concretion in matter, though this is a different type of concretion and one which does not break the unity of the science. Thus within Aristotle's biological works we have a threefold division:

1). The De Anima that treats abstractly of the soul in itself and its universal properties.

2) The Parva Naturalia that treat of the soul as applied to the body and as it concerns all animals, or all living things, or all genera of them.

3) The application of all the above principles to the singles species of plants and animals.

Aquinas also explains what types of problems will arise from the second stage of this procedure. He points out that it has been established in De Anima that the intellectual soul is not in matter, so there can be no concrete treatment of it. Hence the problems concerning it that exceed the scope of natural science pertain to First Science (metaphysics). On the other hand problems concerning the vegetative faculties must be treated concretely as Aristotle did in his Parva Naturalia that Aquinas lists with the mistaken inclusion of a lost spurious De Nutrimento et Nutribili.

In the De Sensu Aristotle begins by recalling the final causes or functions of the external senses. An animal needs senses to preserve its body as a tempered composite of the elements easily destroyed by unfavorable external forces and to obtain the food it requires for sustenance. Consequently all animals have the sense of touch needed to show them the immediate presence of favorable and unfavorable forces and of what may be available as suitable food. Taste assists touch in this determination of appropriate food. The more perfect animals also sense not only what they immediately contact, but also objects at a distance through some appropriate medium. As regards nutrition this requires the sense of smell, but as regards both nutrition and other harmful or helpful things it requires sight and hearing.

Aristotle then compares these senses of sight and hearing to show how for the higher animals that have instincts these two senses are necessary. Such animal instincts are similar to human prudence that guides our action in changing situations but differ from such human prudence in that animal instincts are based only on concrete particulars while human prudence is based on universal truths. In this respect sight is superior to the other senses in that it show us the most distinct and characteristic facts about things, but hearing is also important because it makes it possible for both animals and humans to learn from others.

Aristotle next attempts to state the material causes of the five organs of sensation, deducing them from their final causes or functions that he has already determined. His treatment is very sparse, no doubt because any detailed discussion of the parts of the sense organs would require a study of comparative anatomy not possible until a later stage of medical science. He points out that the ancients such as Empedocles and Democritus attempted to show a correlation between the sense organs and the five elements, but their explanations lacked a critical method.

Aristotle's own research on these questions was hampered both by his primitive four hot-cold and-wet-dry chemical elements and by his unfortunate hypothesis, based however on empirical arguments derived from the medical doctors of his time, that it is the heart rather than brain that is the central organ of the body and hence of the internal senses. Aquinas' accepts this hypothesis although he knew that after Galen (third century AD) this hypothesis had become very much questioned by physicians.

I will give only Aristotle's treatment of sight to illustrate his methodology. He argues that the organ of sight must be such as to receive light from a distant object through the medium (diaphane) because only through such a medium can light be transmitted. Hence, since of the four elements, fire, air, water, and earth of Aristotle's chemistry water seems best suited to be such a transparent medium, he argues that the organ of sight must be composed chiefly of water. To support this argument Aristotle had to refute the ancient view according to which sight depends not on rays received by the eyes, as now know it to be, but on rays emitted from the eye, somewhat like modern radar. This view was supported by the mathematicized optical science of Aristotle's day, Aquinas supposes, because perspective diagrams used by artists often showed a pencil of rays leaving the eye. But such a merely mathematical theory does not meet the requirements of strictly physical theory and hence Aristotle argues that all observable evidence is in favor of the conclusion that light enters not leaves the eye.

That light is transmitted through a transparent medium has been previously established in the De Anima. That its medium of transmission must be water is established by excluding the only other possibility in the four-element chemistry, namely that the medium is air, because (a) air is too variable to form a bodily organ and (b) an examination of the eye shows it does in fact contains water. Aristotle confirms this argument through material causality by reference to the experience of those who have suffered an injury to the eye since the fact that sight is harmed by a trauma that injures only the back of the eye shows that the that passage of light from the front to the back of that organ is necessary for vision.

From similar arguments Aristotle argues that the organ of smell must also be composed of water, but not water as it is transparent, but as it is cold and therefore potentially hot, since it is observed that odors are generated when substances are heated. That the organ of hearing is composed of air had already been demonstrated in the De Anima. In the De Sensu the organs of touch and taste are then shown to be composed of earth, since only earth moistened with water is potential to all the qualities sensed by them.

From these arguments Aristotle draws the general theorem that a sense organ must be composed of the kind of matter that is potential to what the sense actually becomes in the act of sensation, namely, the kind of matter most contrary in quality to the object of each sense. Thus from the final cause or function of an organ of sense, that is, to become actually what its subject already is potentially, one can deduce the matter of which the organ is composed, since the final cause of the organ is the same as the formal cause of its matter.

Given this principle and some examples of its application, the next step is study the sensible objects, since on our understanding of them will depend our deduction of the matter of the sense organs which are potentially similar to these organs. Aristotle then considers the objects of each of the senses and the medium through which each acts on the organs of sight, taste, and smell. He does not treat of touch, since this was extensively treated in the De Anima as the primary sense basic to the others nor does he treat of the sense of hearing because, as St. Thomas remarks (lect 6, n. 78), the generation of its object, namely sound, is exactly of the same character as the change that it produces in the organ of hearing, namely, a vibration of the air and hence its treatment in the De Anima is also sufficient.

Thus in his discussion of the matter of the external sense organs Aristotle proceeds in an entirely empirical manner although always relating this teleologically to the observed function of the organ. That his results are not very satisfactory is principally due to the deficiencies of his chemistry. Since for him the elemental physical bodies are earth, water, air, and fire and the fundamental forces by which they act are the hot and cold, the wet and dry, he fell far short of modern chemistry with its some 100 elements and its four fundamental forces of gravity, electromagnetism, the weak and strong nuclear forces. Less easy to account for is his omission of any an anatomical analysis according to formal causation of the structure of these organs, except in dealing with the problem of the media through which the objects of sensation affect the sense organ, but, of course, such avialable data was sparse until a more extensive use of vivisection prevailed in medicine.

In the case of sight, which again I take as exemplary, its proper object is color, Aristotle defines this with the rather obscure phrase "the limit of the diaphane in determinately bounded bodies" (439b 12). St. Thomas explains (lect 6, n. 87) that this is a definition of an accident abstractly considered and in such definitions it is the subject of the accident that is placed as the difference dividing the genus, in this case, "object of sensation." Thus light in general has already been defined in De Anima as "the act of the diaphane" (i.e., light is as form to the diaphane as matter) and this definition becomes the definition of color in a strict sense when applied to a particular subject, namely to opaque bodies. This is because an opaque body perceives light only at its surface where it contacts the diaphane. The interior of the body is potentially colored, but only the surface is actually so. Transparent or diaphanous substances, such as air or water, are not colored in the strict sense, since they do have such a surface; hence their color changes according to our position within them in relation to some body viewed through this medium.

After this generic definition of color Aristotle considers it species. White and black are respectively the positive and privative extremes. Hence the problem is to explain the intermediate colors of the spectrum. He first considers two hypotheses: either the colors are formed by the juxtaposition of very small particles of black and white so that the different hues are given by the ratios of these extremes with the simple ratios producing the most pleasing colors; or the different colors are formed by an overlay of black on white or vice versa with the result that the various hues are produced by the ratio between these extremes. Aristotle rejects both these hypotheses because they would only produce the appearance of color when the object was not clearly seen and this seems to beg the question. He proposes his own explanation as an application of his theory of chemical mixtures given in De Generatione et Corruptione. In a true mixture the substances are not merely juxtaposed but blended and tempered to form a unified new substance. Similarly their colors can be so combined as to produce new intermediate colors. Thus the hypothesis that these colors have harmonic ratios should be accepted. Aristotle, therefore, on the question of the object of sight concludes that (a) light is received in the eye, not remitted from it; (b) color is a quality actualized only at the surface of a body; (c) colors form a continuous spectrum between black and white in which the mixtures that have harmonic relations (simple ratios) are the most pleasing.

In the last two chapters of the De Sensu et Sensato Aristotle deals with three difficulties. The first of these concerns the divisibility of the sensible qualities and he answers that they are infinitely indivisible potentially, but not actually. They therefore have minimal actual parts that taken separately are only potentially sensible but taken together are actually sensible. The second problem is whether the sensible is first in the medium before it is in the sense. That this is the case for hearing and smell is evident since they take time to travel through the medium. The media of taste and touch, on the other hand, are not extrinsic to the body but are constituted by its own flesh. As for light Aristotle and Aquinas were puzzled by what they mistakenly but understandably believed to be its instantaneous transmission. Aristotle concluded that, since light can instantaneously alter a small part of the medium, a light of sufficient power can do so for the entire medium of any dimensions. The third question is whether we can have two sensations at the same time. He concludes that the common sense, treated in the De Anima, makes this possible since it judges all the senses in a synthetic act.

In the Memoria et Reminiscentia Aristotle first points out that memory and recall are distinct abilities since it is observed that some people have good memories but poor recall and vice versa. He first treats of memory, defining it, obviously enough, as "the apprehension of the past". That this apprehension is an operation of a sensitive faculty is proved by the fact that in the Physics he showed that magnitude, motion, and time are correlative; therefore since magnitude and motion are all included in the object of imagination and the common sense, so must the apprehension of past tome pertaining to sensation. This argument is confirmed by the fact that though brute animals lack intellect still some evidently have memory. Memory, however, differs from imagination in that it adds the note of a particular time to the representation of an image: a remembered flower has the note of a particular past time. This is why we sometimes doubt whether we imagine or remember something and why in hallucinations this distinction gets confused.

Aristotle defines recall as "the resumption of a past impression." It takes place by a connection of similarity, contrast, or association and can be deliberate or accidental. From Aristotle's remarks Aquinas collects four mnemonic rules (lect 5, n. 371): things are best remember when (1) they are in a definite order; (2) one pays close attention to them; (3) they are frequently repeated in the same order; and (4) we begin our recall with what is first in that order. Failures in recall, Aristotle notes, are due to the fact that things may have several orders and in physical events chance can intervene. Errors arise from false connection between event and time. The extension of time is remembered not literally but proportionally. This discussion is summed up in three points (lect 9. 398):. 1) memory and recall are different; (2) recall precedes memory in time; (3) many animals have memory, but only man exhibits recall.

The reason for this last fact is that to recall something deliberately requires that we think syllogistically in beginning a recollection in a definite order. That it is not an intellectual act, however, is evident for a number of reasons such as the fact that recall can be painful, that we have difficulty with recall when emotionally disturbed, that we are haunted by tunes and other memories, and that some people with physical disabilities have poor recall as do children and old people. Since Aristotle does not explicitly discuss whether the internal senses are distinct powers of the soul, he does not settle the question whether recall is a different power than sense memory. Aquinas, however, argues they are not since their object is the same.

How important are these two works as sources of Aquinas' thought compared to the De Anima that is certainly a major source of his whole anthropology? In his commentary on the De Anima Aquinas times cites the De Sensu six timesbut not the De Memoria et Reminiscentia. (1) In De Anima, II, lect. 8, 439 in a passage in which Aquinas notes that De Sensu treats of the qualities as objects of sense and the way they affect the senses. (2 and 3) Another two citations (II, lect 16, n. 439. II, lect 10 n. 515) refer to the material cause of flavors. (4) In II. lect. 10, n., 518 Aquinas cites Aristotle's conclusion that the medium of the sense of touch has as its primary organ something internal, close to the heart, while flesh is merely its medium. (5) Aquinas also cites (II, lect 11, , n. 544) the fact that smelling and hearing have some temporal succession; and (6) finally, he notes (ibid. n. 550) that while De Anima treats of the senses in a summary manner, De Sensu adds many details. This last citation clearly indicates that for Aquinas the De Sensu et Sensato is a significant work in completing the analysis of the De Anima. Although Aristotle often speaks about the instincts of animals, none of these three works deals explicitly with the very important internal estimative sense of animals that in humans is the discursive sense (or particular reason) and Aquinas who considers this the highest of the internal senses had to gather his discussion of it from these scattered references.

Another indication of Aquinas' use of De Sensu et Sensato can be gathered from the many other passages in the Summa Theologiae and the Summa Contra Gentiles where the influence of these Parva Naturalia is evident, although it is not always easy to know whether their contribution is distinct from the De Anima. I will mention two examples, the first taken the sense of sight that I have already described. The introduction to the Leonine edition of the Summa Theologiae lists many others and I doubt that even that list is exhaustive.

Aquinas is frequently concerned with the nature of light because for him, as for the Bible, light is an important analogy for the act of the agent intellect as well as for divine illumination. He says (S. Th. I, q. 67, a. 3 a reference to De Sensu, c. 2) that light has no contrary because it is the natural quality of the heavenly bodies but unlike any body has no weight or number (I q. 5, a. 5 ad 2). Thus it is an instrument of the heavenly bodies to produce substantial forms and to make colors actually visible (I. q. 5 a.5 ad 3). Hence, in I. q. 12, a. 5, ad 1 he argues by analogy that the human agent intellect is necessary as a natural light if we are to receive the supernatural light of the beatific vision. The reason is that light is necessary for sight because it makes the medium transparent in act so that the sense organ can be moved by color. Yet, he notes, this analogy fails in that while physical light makes the object visible, the light of the beatific vision does not make God intelligible since he is that per se but instead empowers the human intelligence in the manner that a faculty is made more able by a habit. Again in S. Th., I: q. 79, a.3 ad 2 Aquinas meets the objection that there is no need for the "light" of an agent intellect, since the purpose of light is to make the medium transparent, by saying that whatever may be the function of corporeal light in sensory vision, the agent intellect is a "light" only in an analogous sense that must not be pressed in every respect (De Anima III, c. 430 a 10-17).

A second example based on a passage in De Sensu c. 6 is found in S. Th., I-II , q. 113, a. 7, c) where Aquinas answers those who doubt that grace can be can be infused instantaneously, by saying that the only reason a form cannot be instantaneously impressed on a subject is because that subject its insufficiently disposed so that the agent needs more time to dispose it properly. Hence we observe that when the matter is well disposed through previous alteration the substantial form is immediately acquired. Similarly when the diaphanous medium is properly disposed to receive light it is immediately illuminated by a radiant body in act. (see also I, q. 53, , a 3, c and III, a 1, ad 3)

In the Summa Theologiae there are also tacit references to the De Memoria et Reminiscentia. Thus in I, q. 27 1 c. the distinction between processions ad intra and ad extra of the acts of soul is used as an analogy for the Trinitarian processions. Again In q, 78 a. 4 distinctions are made of the different ways in which the internal senses are ordered to each other, namely, according (1) to the order of nature as perfect is prior to imperfect the intellect is prior to the senses as directing them and also the senses are prior to the nutritive power; (2) to the order of generation and time as the imperfect is prior since nutritive powers come first in generation and the senses are prior to the intellect; (3) to the interrelation of faculties the visible is prior since it reaches superior and inferior bodies, while sound is prior to odor since it is in air, while odor is mixture of elements. There are also references in the Summa Contra Gentiles as when in III, c.120? or 121, n.2943 it is noted that the inferior powers of the soul are ordered to the higher and in III q. 40, 3 that brute animals have the passions of hope and despair but not as these are moved by an interior intellect but only as they are moved by natural instincts. Also it observed that animals are moved by what they see at a given time to follow or avoid something in the future. From such examples of explicit or implicit citation it is evident that Aquinas' study of the De Sensu and the De Memoria were a significant source of his mature thought.

What then can we conclude from this brief exploration of these two works as sources of Aquinas' thought? First, they reinforce Aquinas' Aristotleian determination to do justice to the role of the body and of sense cognition in epistemology. It is in this respect especially that Aquinas made a unique contribution to the history of philosophy and theology by breaking with the Platonic tradition that had prevailed up to that point in seeking to ground knowledge in innate ideas rather than in empirical observation and was again to prevail in modern philosophy after Descartes. Even in our time noted Catholic thinkers like Karl Rahner and Bernard Lonergan seem never to have freed their thought from this Platonic-Cartesian "angelicism."

Second, it is evident that Aquinas' used certain features of sense cognition as the analogy taken from our direct physical experience to illumine the nature of human spiritual, intellectual knowing, that can be adequately dealt with only in Metaphysics under a broader but analogical concept of ens commune.

Third, the care that Aquinas gave to such questions that pertain to biology and psychology should show Thomists that they need to study them today in the light of our much more developed biology. To continue to rely only on "metaphysical" arguments rather than the natural science arguments used by Aquinas himself is a serious barrier to any Thomistic effort to mediate the Catholic tradition to modern culture.

1. St. Louis, MO: The Modern Schoolman, 1952

2. "La place du De Anima dans le système Aristotélicienne d' après S. Thomas", Archives d'histoire doctrinale et littéraire du moyen age,"Sixième année 31 (published 1932) pp. 25-47.

3.

De Sensu et Sensato I, lect. 1, 2, edition Spiazzi. (Turin/Rome: Marietti, 1949).

4. See the table in P. G. Meersseman, O.P., Introductio in Opera Omnia B. Alberti Magni, O. P. (Bruges: Beyaert, 1931, pp. xi-xii. St. Albert's own paraphrase of these two works is much more elaborate and extensive than that of St. Thomas" and is still available only in vol. IX of the Borgnet edition (Paris: Vives, 1890), pp. 1-118.

5. See the Spiazzi edition of De Sensu, p.4 f. note 4.

6. Festugiere op. cit. thinks that the Historia Animalium is outside Aristotle's own order.

7. Only that on the Athenian Constitution has survived.

8. Jean-Pierre Torrell, O.P., St. Thomas Aquinas, vol. I, The Person and His Work (Washington DC: The Catholic University of America Press, 1996), p. 341.

9. See In Aristotelis Libros De Sensu et Sensatio, De Memoria et Reminiscentia Commentarium, edited by Raymund Spiazzi, O.P. (Turin/Rome: Marietti, 1949), Preface, pp. v-vi. Ivo Thomas, O. P., however, in the translation of the De anima, entitled Aristotle's De Anima in the Version of William of Moerbeke and the Commentary of St. Thomas Aquinas by Kenelm Foster, O. P. and Silvester Humphries, O.P. (London, Routledge and Kegan Paul, 1951, p. 18 argues for a date for that work "as late as 1271", p. 18 and presumably the commentaries on the two shorter additional works would have followed.