Kelly A. Parker
The Ascent of Soul to Noûs: Charles S. Peirce as Neoplatonist
To Real Being we go back, all that we have and are. . . (Enn. VI 5  7)
If there is one project definitive of recent Western philosophy, it may be the search for an alternative to the materialistic metaphysics that has come to prominence with the rise of science. While some insist that the end of metaphysics is the only valid alternative, others call instead for a thorough reconstruction of metaphysics. Such a reconstructed metaphysics must both accommodate the insights of modern science and account for the deeply felt sense that non-material mind or spirit is a real aspect of the cosmos.
The task of reconstructing metaphysics along these lines is enormous. There are only two directions in which to look for the kind of insights that will guide such a project: we may look outside the West to other religious and philosophical traditions, or we may look back into the West to religious and philosophical thought that has been eclipsed by materialism. This last approach inevitably brings one to examine Neoplatonic thought. The question arises, though, how Neoplatonism can accord with modern science. The work of the American philosopher Charles S. Peirce (1839-1914) may be seen in part as an attempt to place the enterprise of modern science into a philosophical framework that is properly (though not exhaustively) described as Neoplatonic.
Part I of this paper traces lines of possible Neoplatonic influences on Peirce's thought. Though there are no obvious direct influences, there are sufficient indirect connections to justify a reading of Peirce as a modern Neoplatonic philosopher. Part II presents some "Neoplatonic" features of Peirce's philosophical system. Part III elaborates these features by comparing Peirce's cosmology to that of Plotinus. Part IV identifies additional significant similarities and differences between Peirce and Plotinus, and proposes further consideration of Peirce's philosophy as a model for "scientific Neoplatonism."
I. Neoplatonic Influences on Peirce's Thought
At the beginning of what is probably his most "Neoplatonic" work, "The Law of Mind," published in the Monist in 1892, Peirce offers the following apologia for what he is about to spring upon his readers:
I may mention, for the benefit of those who are curious in studying mental biographies, that I was born and reared in the neighborhood of Concord -- I mean in Cambridge -- at the time when Emerson, Hedge, and their friends were disseminating the ideas that they had caught from Schelling, and Schelling from Plotinus, from Boehm, or from God knows what minds stricken with the monstrous mysticism of the East. But the atmosphere of Cambridge held many an antiseptic against Concord transcendentalism; and I am not conscious of having contracted any of that virus. Nevertheless, it is probable that some cultured bacilli, some benignant form of the disease was implanted in my soul, unawares, and that now, after long incubation, it comes to the surface, modified by mathematical conceptions and by training in physical investigations. (CP 6.102)
Peirce's disparaging tone in this passage offers a hard beginning to our search for Neoplatonic sources in his thought. This is amplified by the fact that the index of the eight volume Collected Papers reveals no other places where he refers to Plotinus by name. Likewise, a search of Peirce's unpublished papers turned up no signs of direct Plotinian influence.
The late Max Fisch, whose word on these matters is nearly infallible, writes only that the influence of Plotinus and other Neoplatonists on Peirce's mature philosophy is "largely indirect" (Fisch 241-42). Indirect influences can be as powerful as direct ones, of course, and my position is that "some benignant form" of Neoplatonism did animate his work. Peirce devoted most of sixty years to articulating a system of knowledge, based on a rigorous logic and methodology of inquiry, that would encompass all that is humanly knowable about reality. The capstone of his system -- and in his estimation, the highest ambition of the human mind -- are a metaphysics and religious philosophy that reconcile the spirit of modern science with the spirit of ancient philosophy. In evaluating his own relationship to the tradition, Peirce characterized Aristotelianism as "a special development" of Platonic philosophy, and counted himself "an Aristotelian of the scholastic wing, approaching Scotism, but going much further in the direction of scholastic realism" (CP 5.77n1).
This reminds us that what appears as a Neoplatonic influence on Peirce may as easily be the result of influences common to Peirce and Neoplatonism. Indeed, Peirce was a careful student of the works that shaped Neoplatonism. He asserts that by 1894 he had "read and thought more about Aristotle than about any other man" (MS 1604, quoted in Fisch 240). He later turned to the serious study of Plato -- a study he began by copying the Greek texts of key dialogues into notebooks and making his own interlinear translations. He came to consider the Parmenides and the Theaetetus as Plato's greatest works, and went so far as to reconstruct the chronology of the dialogues in the course of a survey of ethical concepts. This exercise notably shaped Peirce's conception of the summum bonum, the Good that defines the telos of all activity in the cosmos (Fisch 240-41). Peirce also investigated Pythagoras' life as a test case for his own logic of drawing history from ancient documents (MSS 476, 1277-78, 1582), and characterized Pythagoras' as "the sublimest of all human biographies" (Fisch 239). On the other hand, Peirce's principle of tychism (absolute chance), which runs so strongly against Neoplatonic principles, was clearly inspired by his reflections on Aristotle and Epicurus (Fisch 231).
It must be noted that Peirce's reading of ancient philosophy was inevitably colored by the traditional interpretation of the texts, and that the traditional interpretation is itself largely Neoplatonic. This effect of the received interpretation of the ancient texts is then the first likely source of indirect Neoplatonic influence on Peirce's thought.
As for traceable Neoplatonic influences, there is an obvious line of contact that needs to be explored further. Peirce was above all a logician, and had read widely in the available Neoplatonic and Stoic sources. References to Porphyry are frequent. All such references that I have checked, however, seem confined strictly to logical concerns, even where the originals clearly carry a metaphysical burden. Peirce would not have been incognizant of that burden, though, so we have here a second likely source of indirect influence.
Finally, Peirce was of course conversant in more recent philosophy. He knew and respected the work of such latter-day Neoplatonists as Leibniz and Herbert Spencer. They must not be overlooked as likely sources of Neoplatonic themes in Peirce's work.
I suggest that the notable similarities between the Peircean and Neoplatonic philosophies are due not primarily to particular direct or indirect Neoplatonic influences, but rather to the philosophers' similar aims, to shared ancient sources, and to the residual effects Neoplatonism had on the traditional interpretation of these sources. For these reasons it is legitimate to consider Peirce as a metaphysician and cosmologist working within the Neoplatonic tradition, though perhaps he did not do so self-consciously.
II. Plotinian Themes in Peirce's Philosophy
The signature feature of Peirce's philosophy is his theory of three irreducible and universal categories. The categories appear in different guises depending on the area of thought under consideration. In mathematics and logic they are forms of relation; in phaneroscopy (the analysis of phenomena) they are elements of any experience; in metaphysics they are modes of being. Since these universal categories first appear in mathematics, and because they "are best defined in terms of numbers," Peirce sometimes calls them the "kainopythagorean categories" (CP 7.528). He defines the phaneron as "the collective total of all that is in any way or in any sense present to the mind, quite regardless of whether it corresponds to any real thing or not" (CP 1.284). All three categories are inevitably to be found in any phaneron. They are thus similar to the five genera or categories identified in Plato's Sophist (254-55), categories which Plotinus took to be the primary categories of the intelligible world (Enn. 6  8). The closest parallel to Peirce's categories in Plotinus is probably the identification of Being, Life, and Thought as aspects of Noûs (Armstrong 246).
Peirce's first attempt to identify the categories, based on the analysis of propositions, produced a list of five: Being, Quality, Relation, Representation, and Substance (W 2:49-59). Peirce shortly revised the list to three categories, on the basis that Being and Substance are the unthinkable limits of cognition, and hence have no meaning in thought. He also later renamed the three remaining categories as generically as possible.
Firstness is the monadic mode -- uncaused, irrational possibility; spontaneous occurrence. Secondness is the dyadic mode -- the brute fact that something just is, as it is, in relationship to others. Thirdness is the triadic mode -- the meaning of a phaneron for a mind; the habit, law, or pattern that emerges as unifying many particular actions for an observer; the general idea that encompasses an infinite number of individuals under a class (CP 1.284-353, 5.41-119). In discussing the three categories, it is crucial to note an implicit principle that emerges in the tension of Firstness' spontaneity and Secondness' stability: the world manifests change and stasis everywhere, at all levels. The fractal images discussed in David Fideler's contribution to this volume nicely represent this interplay between Firstness and Secondness, and Peirce would likely point to the fact that we so quickly learn to look for a recurring pattern in these chaotic sets as an illustration of Thirdness in our interpretation of the images.
Peirce's pragmatism stipulates that the meaning of any idea consists in "our concept of what our conduct would be upon conceivable occasions" in which the idea might enter (CP 8.208). This is the definition of meaning that led him to amputate the "false categories" of Being and Substance from his list: these terms name modalities beyond the limits of relation, and hence beyond conception. With respect to Plotinus's hierarchy, Peirce would no doubt eliminate both matter and the One for the same reason. Of the Plotinian cosmos, then, Noûs, Soul, and Bodies would remain for Peirce's philosophical consideration. This does not necessarily imply that the One is unreal in Peirce's scheme, but perhaps only that the ineffable is ineffable and that discourse and thought have no dealings There: philosophy, for Peirce, is evidently a dialectical activity of the soul. Within the remaining Plotinian realms, each of Peirce's three categories would be found. The most obvious correlation, to my eyes, is that in Noûs, where Life exemplifies Firstness, Being exemplifies Secondness, and Thought exemplifies Thirdness. This correlation, however, is merely an illustration of how Peirce's categories relate to Plotinus' hypostases. For real insight, we turn to Peirce's own words on cosmology.
III. Peirce and Plotinus on Cosmology
Peirce's philosophy emphasizes the principles of continuity and process. His semeiotic, or general theory of signs, is an all-purpose model of continuous process. I cannot here go into the details of Peirce's semeiotic, but will only note that in his system metaphysics and cosmology are based upon the results of logic, where logic is conceived as semeiotic. His metaphysics, for which he preferred the name synechism, combines logical realism with metaphysical idealism. Mind, which is not an individual thing for Peirce, "lives, and moves, and has its being" in the continuum of symbols (NEM 4:344). The cosmos embodies a logic that our own can only approximate, and the whole of what there is can be seen as the unfolding of an evolutionary process describable as cosmic semeiosis: "all this universe is perfused with signs, if it is not composed exclusively of signs" (CP 5.448n1). Signs are teleological. By implication, so is the universe, and Peirce ventures to state the telos of the universe in these terms:
The purpose of every sign is to express "fact," and by being joined with other signs, to approach as nearly as possible to determining an interpretant which would be the perfect Truth, the absolute Truth, and as such . . . would be the very Universe. Aristotle gropes for a conception of perfection, or entelechy, which he never succeeds in making clear. We may adopt the word to mean the very fact, that is, the ideal sign which should be quite perfect. . . . The entelechy of the Universe of being, then, the universe quâ fact, will be that Universe in its aspect as a sign, the 'Truth' of being. (NEM 4:239-40)
The universe is an argument of vast scope, whose end is not a determinate conclusion statable in a proposition (since propositions serve the higher end of expressing truth), but is rather the living idea that is reality. The universe is an ever-growing continuum in which thought, matter, and feeling are coming to be welded together into a harmonic state of "concrete reasonableness."
Peirce writes that "Synechism is founded on the notion that the coalescence, the becoming continuous, the becoming governed by laws, the becoming instinct with general ideas, are but phases of one and the same process of the growth of reasonableness" (CP 5.4). One might well ask why Peirce thought the cosmos is striving toward greater reasonableness. The answer is that his best hypothesis as to the origin of the cosmos indicates that it must necessarily be doing so, in the long run.
Peirce's cosmology, or "mathematical metaphysics" (CP 6.213) aims to show "how law is developed out of pure chance, irregularity, and indeterminacy" (CP 1.407). The account, outlined in the accompanying chart, unfolds as follows.
If we are to proceed in a logical and scientific manner, we must, in order to account for the whole universe, suppose an initial condition in which the whole universe was non-existent, and therefore a state of absolute nothing.
. . .
But this is not the nothing of negation. . . . The nothing of negation is the nothing of death, which comes second to, or after, everything. But this pure zero is the nothing of not having been born. There is no individual thing, no compulsion, outward nor inward, no law. It is the germinal nothing, in which the whole universe is involved or foreshadowed. As such, it is absolutely undefined and unlimited possibility -- boundless possibility. There is no compulsion and no law. It is boundless freedom.
Now the question arises, what necessarily resulted from that state of things? But the only sane answer is that where freedom was boundless nothing in particular necessarily resulted.
. . .
I say that nothing necessarily resulted from the Nothing of boundless freedom. That is, nothing according to deductive logic. But such is not the logic of freedom or possibility. The logic of freedom, or potentiality, is that it shall annul itself. For if it does not annul itself, it remains a completely idle and do-nothing potentiality; and a completely idle potentiality is annulled by its complete idleness. (CP 6.215-219)
Thus the principle that the logic of the universe is at least as sophisticated as our own -- that it therefore includes retroduction or abduction, the spontaneous form of inference that initiates a stream of inference -- leads us to an account of the first stirrings of determination in the utter indeterminacy of Nothing. This is the first appearance of a mode of positive possibility, different from the mere absence of determination that characterizes the initial zero-state.
I do not mean that potentiality immediately results in actuality. Mediately perhaps it does; but what immediately resulted was that unbounded potentiality became potentiality of this or that sort -- that is, of some quality.
Thus the zero of bare possibility, by evolutionary logic, leapt into the unit of some quality. (CP 6.220)
The potentiality of a quality, in Peirce's metaphysics, is analogous to the Platonic Form or Idea, in that it is a timeless, self-subsisting possibility that serves as the metaphysical ground of the world of actual existence.
The evolutionary process is, therefore, not a mere evolution of the existing universe, but rather a process by which the very Platonic forms themselves have become or are becoming developed. (CP 6.194)
Peirce says that the very earliest stages of cosmological evolution are characterized by extreme vagueness of form (CP 6.191). Our hypothetical single, spontaneously emerging qualitative possibility is not sharp-edged; the process of determination as yet consists only in a lone arbitrary determination within the zero-state. This is the possibility of a quality, indeed, but a quality that is not distinct from any other qualities: it thus brings a whole continuum of immediately connected possible qualities into being. The process has to unfold further before qualities can meaningfully be considered distinct.
[W]e must not assume that the qualities arose separate and came into relation afterward. It was just the reverse. The general indefinite potentiality became limited and heterogeneous. (CP 6.199)
The evolution of forms begins or, at any rate, has for an early stage of it, a vague potentiality; and that either is or is followed by a continuum of forms having a multitude of dimensions too great for the individual dimensions to be distinct. It must be by a contraction of the vagueness of that potentiality of everything in general, but of nothing in particular, that the world of forms comes about. (CP 6.196)
With the emergence of the continuum of positive possibility, the first of the three "universes of experience," the Universe of Ideas or Possibility, is established (CP 6.455).
There is, however, an element of Secondness in the emergence of the continuum of forms where there was only indefinite nothingness before, and an element of Thirdness in the continuity and eternal subsistence of those forms. As the evolution continues, Secondness comes to the fore. Nascent relations of identity and difference emerge in and among parts of the continuum of forms, and qualities thereby come to be differentiated.
The second element we have to assume is that there could be accidental reactions between those qualities. The qualities themselves are mere eternal possibilities. But these reactions we must think of as events. Not that Time was. But still, they had all the here-and-nowness of events. (CP 6.200)
The next milestone in the evolution of the cosmos is the appearance of enduring existence, the Universe of Brute Actuality of things and facts (CP 6.455). The evolutionary shift from the first universe to the second, however, is not abrupt: what Peirce is describing is a continuous process of beginning. The designation of the different universes only indicates stages in the process. As the development of relations progresses, through several stages of evolution by chance occurrence, time and space emerge.
Out of the womb of indeterminacy we must say that there would have come something, by the principle of Firstness, which we may call a flash. Then by the principle of habit there would have been a second flash. Though time would not yet have been, this second flash was in some sense after the first, because resulting from it. Then there would have come other successions ever more and more closely connected, the habits and the tendency to take them ever strengthening themselves, until the events would have been bound together into something like a continuous flow. (CP 1.412)
This continuous "quasi-flow" represents the headwaters of the almost perfectly continuous temporal stream that we experience.
The quasi-flow which would result would, however, differ essentially from time in this respect, that it would not necessarily be in a single stream. Different flashes might start different streams, between which there should be no relations of contemporaneity or succession. So one stream might branch into two, or two might coalesce. But the further result of habit would inevitably be to separate utterly those that were long separated, and to make those which presented frequent common points coalesce into perfect union. Those that were completely separated would be so many different worlds which would know nothing of one another; so that the effect would be just what we actually observe. (CP 1.412)
Space develops in a similar fashion, through the spontaneous occurrence of pairs of events that tend to become ever more regular under the influence of the principle of habit. Substance, the distinguishing feature of an enduring thing, develops out of the establishment of habits within such pairings.
Pairs of states will also begin to take habits, and thus each state having different habits with reference to the different other states will give rise to bundles of habits, which will be substances. Some of these states will chance to take habits of persistency, and will get to be less and less liable to disappear; while those that fail to take such habits will fall out of existence. Thus substances will get to be permanent. (CP 1.414)
In the nascent time and space, then, organized bundles of habits come to embody particular qualities that are distinguishable from other bundles of habits embodying other qualities. Those bundles of habits that tend to persist, which happen to acquire the habit of enduring, come to dominate the universe in the form of existing things or actualities. Thus the cosmos develops into a state where Secondness predominates, and this Peirce calls the Universe of Actuality (CP 6.455).
Peirce next turns to the principle of habit-taking, and its most dramatic effect on the evolving cosmos:
all things have a tendency to take habits. . . . [For] every conceivable real object, there is a greater probability of acting as on a former like occasion than otherwise. This tendency itself constitutes a regularity, and is continually on the increase. . . . It is a generalizing tendency; it causes actions in the future to follow some generalizations of past actions; and this tendency itself is something capable of similar generalizations; and thus, it is self-generative. (CP 1.409)
The principle of habit-taking, operative but feeble in the first two stages of cosmic evolution, begins to come to prominence once actuality is well established. Actuality is characterized by reactions among enduring things. The character of such things, and consequently the relations and modes of interaction among them, would be extremely irregular at first. The principle of habit-taking has the effect of making events in the Universe of Actuality more stable and regular. It underlies the emergence of permanent substances, as we have seen. Beyond this, it has the effect of stabilizing the kinds of reaction which tend to occur among different substances. Nothing forces there to be a tendency toward regularity in the Universe of Actuality, for the notion of force implies necessity, an advanced variety of the regularity we are trying to explain (CP 1.407). Regularity, like possibility and particularity, must appear in the evolving cosmos by chance. But just as we have seen the tendency to take habits operate on Firstness to establish the Universe of Ideas and on Secondness to establish the universe of Actuality, so does it operate on Thirdness, on itself, to establish a universe dominated by Thirdness, lawfulness, order, and reasonableness.
The element of habit-taking has a peculiarity about it: it is the only possibility that, once occurring, "can grow by its own virtue" (CP 6.101). Thus it operates in the respective universes with increasing efficiency. In the Universe of Actuality it serves to establish regularity in the various substances, both in their internal nature and in their external relations to one another. It gives rise, in short, to the regularities we recognize as the laws of nature. According to Peirce's cosmology, "each law of nature would consist in some permanence, such as the permanence of mass, momentum, and energy. In this respect, the theory suits the facts admirably" (CP 1.415). As the principle of habit-taking operates to promulgate its own influence, events in the actual world become more regular, more predictable. Necessity -- as embodied, for example, in the idea of necessary connection which Hume rightly said is absent from the observable realm of existent things -- emerges in the form of laws that govern events. Necessity is the support of actuality by reason, the generalization of the particular events in the Universe of Actuality (MS 277, p. 123).
As law takes hold, the evolving cosmos can be seen wending its way toward a Universe of Necessity, in which law would be perfect. This Universe is the completely reasonable state of things that is identified (in Peirce's esthetics) as an ideal. This universe is on the one hand unrealizable in principle, since it would imply the complete eclipse of Possibility and Haecceity, which are as fundamental as Necessity; on the other hand, it is the regulative ideal toward which self-controlled thought and action aim. The increase of reasonable thought and action, in the context of all three universes, is accordingly the summum bonum in Peirce's philosophy.
This rather lengthy account of Peirce's cosmology should suggest a number of things to the reader already familiar with Plotinus' account of creation. At the most general level, it is notable that Peirce, like Plotinus, sees the origin of the universe as the result of a spontaneous (i.e. uncaused) act of creation. This act of creation proceeds through stages, and both philosophers see the realm of existent bodies as an imperfect reflection of the realm of Forms that is the proper object of knowledge. Though the pregnant Nothing of the zero-state bears some affinities to the Plotinian One, there is apparently no room in Plotinus for Peirce's doctrine of tychism, his insistence on irrational chance as the driving force behind creation. The Plotinian One is the ground of creativity and necessity. Peirce separates these two principles. Creativity is Firstness, the generative principle, while necessity is Thirdness, the end toward which events in the universe are drawn. Given the role of indeterminacy in modern scientific explanations, it may be that Peirce's approach is to be preferred.
Like Plotinus, Peirce sees the Platonic Forms as teeming and seething with life (Enn. VI 7  12), but for Peirce this implies that the Forms develop, evolve, and change. Peirce and Plotinus both reject the Platonic notion that the laws of nature are "laid down in advance and then applied" (Armstrong 253). Plotinus holds that they appear fully perfected in Nature when Soul contemplates Noûs; Peirce sees them as imperfect, and perfecting them is itself the telos of the existent universe. Peirce's philosophy, moreover, banishes the One from metaphysics: there is no completed perfection with which we might in fact be (re)united. Peirce does mention belief in a God who is in and yet transcends the cosmos (CP 6.452), but except so far as its immanent manifestations are explored under other areas, this God is not a possible object of our knowledge. The entelechy of the universe is an ideal to be approximated through the infinite, ongoing effort of mind acting in the world we inhabit. For that reason, Peirce does not denigrate material body: the Universe of Existence is for him an indispensable positive aspect of reality, and not a flawed reflection of reality. In a problematic passage, Porphyry says that Plotinus "seemed ashamed of being in the body;" Peirce might ask just how he would ever accomplish much without a body.
Where Peirce and Plotinus are in sympathy, the possibilities are exciting indeed. I will mention only three such areas. First is Peirce's mathematical analysis of the continuum as an infinitely divisible plenum, in which every part has the same properties as the whole. Second is his modelling of this principle of continuity in a theory of signs, which allows us to view the universe as a process of continuous and reflexive semeiosis, every part of which potentially carries us a fragment of greater Truth. Finally, Peirce's related work on the mathematics of transfinite sets (work which diverged from Georg Cantor's in metaphysically important ways) may help resolve some difficulties in Plotinus' conception of infinity.
Peirce's account of the summum bonum, the disciplined development of mind toward a state of perfect order, lawfulness, and concrete reasonableness, seems almost a perfect adaptation of the Plotinian idea that Soul seeks contemplation and union with Noûs. The difference is that Peirce makes refinement of the process, and not its completion, the telos of the activity. In Peirce's philosophy, this process of deliberate and self-correcting pursuit of Truth, the ascent of soul to Noûs, is nothing other than scientific inquiry.
Peirce's metaphysical doctrine of synechism is his account of how things are with us that this progression is possible. For Peirce, we are of the same stuff as the cosmos. Not just materially, but also at the higher level of Form: the human person, in one facet, is a symbol and hence already is a part of that elevated reality. This accords with Plotinus' doctrine that the higher part of the soul already participates in Noûs. This effective continuity of soul and idea, a central corollary of synechism, in fact appears capable of accomodating Plotinus's account of the efficacy of prayer and magic (CP 6.516; Enn. IV 4  40-41).
As we approach the close of the twentieth century, philosophy, science, and religious thought stand in need of a metaphysics and cosmology that do justice to the full human encounter with reality. This encounter includes not only our experience of the ordinary, but also our most advanced scientific inquiries and our most elevated spiritual experiences. The ancient Neoplatonic thinkers sought to express the mystery of our relation to Being itself in their accounts of the soul's journey toward higher reality. In what is here referred to as the "scientific Neoplatonism" of Peirce's metaphysics, this highest human ambition is characterized as the deliberate effort to enter into and express Truth. Peirce urges us to see that this human endeavor is commensurate with, and contributes to, the unfolding of the cosmos itself. Perhaps the account of reality we seek has been here all along, then, in the form of a Neoplatonism that has continued to develop even in the shadow of modern science -- its "cultured bacilli" carried into this century by philosophers such as Charles S. Peirce.
Armstrong, A. H. "Plotinus." Part III in The Cambridge History of Later Greek and Early Medieval Philosophy. Ed A. H. Armstrong. New York: Cambridge University Press, 1967.
Fisch, Max H. Peirce, Semeiotic and Pragmatism. Ed. K. L. Ketner and C. J. W. Kloesel. Bloomington, Indiana: Indiana University Press, 1986.
Peirce, Charles S. The Charles S. Peirce Papers. Originals housed in the Houghton Library, Harvard University, Cambridge, Massachusetts. Copies housed in the offices of the Peirce Edition Project, Indianapolis, Indiana. References to the Papers are given in the form (MS 476), where the number is that assigned to the document in the following catalogues:
Robin, Richard S. Annotated Catalogue of the Papers of Charles S. Peirce. Amherst: University of Massachusetts Press, 1967.
_______. "The Peirce Papers: A Supplementary Catalogue." Transactions 7 (1971): 37-57.
________. Collected Papers of Charles Sanders Peirce. 8 vols. Vols. 1-6 ed. Charles Hartshorne and Paul Weiss. Vols. 7-8 ed. Arthur Burks. Cambridge: Harvard University Press, 1931-58. References to this edition are given in the form (CP 6.102), where the first number indicates volume and the second indicates paragraph(s).
________. The New Elements of Mathematics. 4 vols. Ed. Carolyn Eisele. The Hague: Mouton Publishers, 1976. References to this edition are given in the form (NEM 4:239-40), where the first number indicates volume and the second indicates page(s).
________. Writings Of Charles S. Peirce: A Chronological Edition. 5 vols. to date. Ed. Edward Moore, Christian J. W. Kloesel, et al. Bloomington: Indiana University Press, 1982--. References to this edition are given in the form (W 2:49-59), where the first number indicates volume and the second indicates page(s).
Plotinus. The Enneads. Trans. S. MacKenna. Burdett, New York: Larson Publications, 1992. References to this volume are given in the form (Enn. IV 4  40-41), where the Roman numeral indicates Ennead, first Arabic numeral indicates treatise, the bracketed number indicates position in Porphyry's chronology, and the last Arabic numeral indicates chapter.
Peirce's Cosmology, or Mathematical Metaphysics