Derivation, in the sense of word formation, and inflection, whose distinction dates back to Varro’s concepts of uoluntaria declinatio and naturalis declinatio, in the 1st century B.C., have been characterised by morphologists today as having non-rigid boundaries. Authors such as Haspelmath (1996), Spencer (2013; 2016) or Štekauer (2015) show that criteria used to distinguish derivation from inflection are not universal, since it is possible to find examples in languages that contradict them. Such criteria are based on features such as purpose, syntactic determinism, obligatoriness, productivity, transparency, base inheritance, and exponence, as proposed by Brown & Hippisley (2012: 37). The first criterion – purpose – is related to what Spencer (2016: 37) calls the Lexeme Individuation Problem, which he formulated as “how do know when we have two distinct lexemes rather than two forms of one and the same lexeme?” (2016: 37). Since “building a new lexeme from an existing lexeme” (Brown & Hippisley 2012: 37) is the purpose of derivation, building new lexemes may be one of the criteria to distinguish derivational morphemes from inflectional ones. However, as proven by Brown & Hippisley (2012) and Spencer (2013; 2016), there are intermediate categories, such as participles (e.g., the bird is singing / the singing bird / the singing of the bird), where the criterion may be questioned. The second criterion – syntactic determinism – is related with Varro’s distinction. Inflection is determined by syntax whereas word formation is not determined by syntax. The Lexeme Individuation Problem and syntactic determinism lead us to another question: “how do know when we have an inflectional morpheme or a derivational morpheme?”; or to put in other terms: “how do know when a morpheme is serving syntax or word formation?”. Is the English morpheme -ing a derivational or an inflectional morpheme? Is it serving syntax or word formation? The problem becomes more striking if we bear in mind that one of the possible sources for derivational affixes is inflectional affixes (Rainer 2016). The search for possible answers to the problem constitutes the aim of this paper. To provide answers, we need to rely on i) the distinction between inflection and derivation, and ii) the perspective that this distinction is fuzzy, in the sense of the word fuzzy found in Computing Logic, i.e., that it is dependent on degrees, and that both concepts should be observed as a continuum between two poles. The continuum is defined by criteria. The more criteria a morpheme presents the more derivational it is, i.e., the more it serves word formation. The criteria proposed by this paper are based on a cross-linguistic analysis of the behaviour of suffixes that have an etymological origin in the Latin present participle -NTE- morpheme (e.g., amans, amantis, being AMO ‘to love’ the verb). The history of this morpheme, across languages and time, is of great value to understand the fuzziness between inflection and derivation. The present participle in Latin is part of the verb inflection and is inflected in number and case. Hence, it serves syntax. However, in Latin, it also builds nouns (Ernout & Thomas 1951: 275) (e.g., adulescens ‘young man’, legens ‘reader’). For this reason, it also serves word formation. In Medieval Portuguese, the suffix is an in-between category, functioning either as a verb-inflected form (cobiçante nós põer cima aas demandas ‘being our desire to end the petitions’ (Orden. D. Afonso II)) or as an adjective (estando os filhos presentes e chorantes ‘being the children present and crying’) (Crónica da Ordem dos Frades Menores) (apud Nunes 1919). In the 18th century, the verbal nature of the Portuguese morpheme has disappeared, only remaining in a few lexicalised forms. Adjectives and nouns are preponderant (Bluteau 1712-1728). Due to its verbal origin, the morpheme in Romance languages works mainly in deverbal word formation, either adjectives or nouns. However, the suffix also constructs nouns and adjectives that are not related with a verb base but with a noun or an adjective (e.g., Italian nottante ‘nurse taking care of the sick during the night’ : notte ‘night’; Spanish farsante ‘hoaxer’ : farsa ‘farce’; Portuguese bandeirante ‘explorer of the Brazilian hinterland in conquest of new land, gold and precious stones’ : bandeira ‘flag; expedition’; Romanian familiant ‘member of a family’ : familie ‘family’; furiant ‘kind of dance’ : furie ‘fury’). The morpheme may even provide the derivative with a pejorative meaning that is absent from the base and from the original derivational paradigm (Italian tedescante ‘supporter of Germans or Austrians’ : tedesco ‘German’). These examples show that the morpheme has been acquiring a ‘serving word formation’ status. This status is highlighted in non-Romance languages, where the morpheme etymologically originating from the Latin -NTE works in the formation of words whose bases are not necessarily verbs (e.g., English malignant, benignant, contrariant [Bauer et al. 2013]). From the analysis of the behaviour of the words with suffixes that etymological go back to the Latin morpheme, this paper proposes criteria to classify a morpheme in a gradience between inflection and derivation, i.e., falls between serving syntax or word formation. The ratio between the number of criteria appliable to the morpheme to the number of abstract criteria permits a quantification of the gradience. The proposed criteria are the following: i) change word class; ii) structural constraints; iii) theta-mark via preposition; iv) lack thematic structure; v) admit modifiers different from the modifiers of the base; vi) admit gender variation or has a rigid gender; vii) present a base which does not belong to the same class of words where it works as an inflection morpheme; viii) work in cross-paradigms; ix) occur in languages other than the branch of languages coming from the language of origin of the morpheme; x) semantic shift towards an evaluative semantic feature which is absent from the base. References Bauer, Laurie et al. (2013). The Oxford Reference Guide to English Morphology. Oxford: Oxford University Press. Buteau, Raphael (1712-1728). Vocabulario portuguez e latino. 8 volumes + 2 supl. Coimbra: Na Companhia de Jesu. Brown, Dunstan & Andrew Hippisley (2012). Network morphology: A defaults-based theory of word structure. Cambridge: Cambridge University Press. Ernout, Alfred & François Thomas (1951). Syntaxe latine. Paris, Librairie C. Klincksieck. Maria Lo Duca (2004). Nomi di agente [Derivazione nominali denominale] In Maria Grossmann & Franz Rainer (Eds.), La formazione delle parole in italiano. Tübingen: Max Niemeyer Verlag, pp.191-218. Nunes, José Joaquim (1919). Compêndio de gramática histórica portuguesa. Lisboa: Livraria Clássica Editora. Rainer, Franz (2016). Derivational Morphology. In Adam Ledgeway & Martin Maiden (Eds.), The Oxford Guide to the Romance Languages. Oxford: Oxford University Press, pp. 513-523. Spencer, Andrew (2013). Lexical relatedness: A paradigm-based model. Oxford: Oxford University Press. Spencer, Andrew (2016). Two morphologies or one? In Andrew Hippisley & Gregory Stump (Eds.), The Cambridge Handbook of Morphology. Cambridge: Cambridge University Press, pp. 27–49. Varro, Marcus Terentius (47–45 B.C.). De Lingua Latina, Julius Pomponium Laetum (ed). Rome: Georgius Lauer, s.d. [ca. 1471], editio princeps. This publication was supported by Project UIDB/00022/2020, FCT-Fundação para a Ciência e Tecnologia, Portugal, through national funds. info:eu-repo/semantics/publishedVersion