“Poinsot, so far as present knowledge goes, holds the privileged position in semiotic historiography of being the earliest systematizer of the ‘doctrine of ‘signs. Not until the work of Peirce in our own day do we again encounter a ‘semiotic of comparable energy and scope. In 1632, Poinsot published, as part of his series of courses in philosophy at the University of Alcalâ, Spain, a highly original, systematically conceived Treatise on Signs (Tractatus de Signis) (1930), which fits exactly ‘Locke’s definition of semiotic proposed some 58 years later, at the close of his Essay Concerning Human Understanding (details in Deely 1982), and taken up again by Peirce.
From this point of view, Poinsot’s work provides us with the first of several “missing links” in the history- of logic and ‘philosophy after ‘Ockham (e. 1350), enabling us to trace backwards through the Iberian schools of Coimbra (notably in the work of Petrus Fonseca  and the team of workers he organized, the so-called “Conimbricenses”), Salamanca (Bavez, Soto, and others), and Alcalâ, a heretofore largely untold story of developments that are exceptional in import for semiotics (Deely 1982).
Doctrinally, Poinsot’s work achieves a new, entirely experiential point of departure for the enterprise of philosophy, and reconciles in so doing the seemingly opposed orders of nature and ‘culture. Poinsot begins his Treatise on Signs by drawing attention to a central feature of ‘semiosis that must, in his opinion, be a first concern of semioticians to safeguard and give adequate account of, namely, the fact that, in our experience, signs bring together natural and social phenomena. The sign, he points out (Book 1, Question I: 646b26-45), is something neither preclusively natural nor preclusively social, but both inclusively, for while all signs as such acquire their signification and actually exist only within some living being’s experience, nonetheless, within that very experience, the connection between signs and what they signify sometimes seemsto have roots outside our experience of their connection (the case of “natural” signs), and other times seems to have no reality other than the one derived from the experience itself of social interaction (the case of customary and stipulated signs). Thus the first task of the semiotician, in Poinsot’s judgment, is to secure a standpoint superior to the division of being into what exists independently of our ‘cognition (ens reale ‘mind-independent being’) and what exists dependently upon cognition (ens rationis mind-dependent being’). For Poinsot, semiotic must take its stand, in the felicitous description by Sebeok, squarely “at the intersection of nature and culture.” This simple description of semiotic’s initial task already amounts to a revolution within the perspective of natural philosophy or “physics” traditional in Poinsot’s day. For the sole concern of that tradition was to uncover and explicate the structure of ens reale, which they thought to have achieved, after “Aristotle, with the division of mind independent being into substances, or natural units of independent existence, with their accidents, or various properties and characteristics. Thus, the division of being into the Aristotelian categories of substance and the various types of accident was generally thought to be the permanent achievement of ontology in the Latin age.
Poinsot’s approach to semiotic entirely undercuts this categorial scheme, going beneath it and beginning with an analysis of experience prior to the possibility of the working out of any such scheme. He establishes a fundamental ontology in just that sense which Heidegger calls for in our own time, namely, an “ontology” that accounts for the categorial interconnections and lays bare the ground of the prior possibility of truth as a “correspondence” between thought and being. Poinsot finds this fundamental ontology in our experience of the ways in which things appear to be relative. Poinsot observed (following in this Aquinas [c. 1266: q. 28] and Cajetan  before him) that, as a mode of reality, relation is unique in that its essence (esse ad aliud ‘being between’) is separate from its cause or ground of existence (esse in alio ‘the character or feature upon which a relation is founded’), which is not the case for any other mode of reality. Poinsot sees in this the ultimate reason for the possibility of semiosis: relation in what is proper to it, namely, suprasubjectivity or intersubjectivity (esse ad), is indifferent to realization now in nature, now in thought, now in both. Relation in this sense, precisely as indifferent to the opposition of what depends upon and what is independent of cognition, Poinsot calls relatio secundum esse ‘relation according to the way it has being’ or ‘ontological relation’ (see Deely 1982).
By contrast, things that are related exist subjectively as something in their own right, not just between other things sustaining them in a derivative way. And yet, if we seek to explain why they are as they are or how they might be altered from their present state, we find it necessary to refer to what the individuals in question themselves are not. Thus, even the individual entities and “natural units” of experience existing in their own right – even substances in Aristotle’s scheme, the most absolute of the subjective entities – are seen to be relative when it comes to the question of how they come to be or of how they are to be accounted for. Relativity in this sense, precisely as infecting the whole scheme of categories of cognition-independent existents, Poinsot termed relatio secundum dici ‘relation according to the way being must be expressed in discourse’, or (synonymously) relatio transcendentalis ‘transcendental relation’.
With this division of being, then, into transcendental and ontological relation, Poinsot has two simple “categories” that are exhaustive and exclusive, but whose terms are entirely matters of direct experience (unlike Aristotle’s division of being into substance and accident, which was also exhaustive and exclusive, but directly experienced only on the side of certain accidents: comprehensive discussion in Powell [Freely chosen reality] 1982), and whose relevance to the doctrine of signs is immediate. For all authors agree, and indeed experience makes quite unmistakable, that every sign as such is a relative being (something making known another than itself), and since, by the prior terms of the analysis of relative being, we know that there are only two irreducible types of relativity, it remains only to apply that analysis to our experience of semiosis in order to determine in what precisely a sign consists (the formalis ratio signi, as Poinsot puts it), that is to say, what is it that constitues a sign in its proper being?The answer to this question is ontological relation, an answer which enables Poinsot to resolve a number of aporia that have plagued accounts of signifying from ancient times down to the present, and which turn out to be decisive for °epistemology and philosophical thought generally.”
From: John Deely – Poinsot, John – in: Encyclopedic dictionary of semiotics – General Editor, Thomas A. Sebeok; Editorial Board, Paul Bouissac … [et al.] – Berlin, New York, Mouton de Gruyter, 1986.