A Study of William of Ockham's Logic - from Suppositio to Truth Conditions Catarina Dutilh Novaes Abstract: This work is the result of my attempts to combine my Philosophy background with the Mathematical Logic inclinations of the institution within which this research was developed. In fact, this twofold character is noticeable in many features thereof; I shall now outline some of them. The project has two main purposes: the less risky one is to provide an account of William of Ockham's logical thinking, with focus on its aspects which bear a relation with the contemporary issues of intensional logics and possible-world semantics. The more risky one consists of investigating the possibilities of developing a purely extensional treatment of intensional contexts, such as tense and modalities. For the latter, some other extensional/nominalistic systems could have played the role of `experimental sample', but there seemed to be something intriguing about Ockham, as one wonders whether a philosopher from the XIVth century would have something relevant to add to our present logical issues. Moreover, he is considered to be the founder of nominalism, so the historical interest of such enterprise was self-evident - therefore, the legitimacy of the first purpose. I shall try to comply with two very distinct kinds of expectations: those which are the desiderata for a History of Philosophy work, and those of logicians, who are interested in the formal correcteness of the system hereby presented. The criteria of excellence of these two lines are almost incompatible, and one wonders if it is not a suicidal enterprise to try to combine them. On the one hand, an Ockham scholar may be discontent with the absence of a few important aspects of Ockham's logic, since I deliberately priorize those related to contemporary logic. On the other hand, a logician may be bothered by the presence of too many `antiquities', perhaps hindering logical clarity. So, at the risk of displeasing everybody, I nevertheless maintain that such a combination may turn out to be fruitful and informative to both sides. Chapter 1 will display some fundamental aspects of Ockham's logic and semantics, in a rather historical approach. However, even this part is developed taking into account what I later shall want to establish as my version of an `ockhamist system'. I consider it to be the flaw of many such reconstructions that they do not undertake a serious analysis of the underlying concepts; alternatively, some which did rely on such an analysis have reached very interesting results. Chapter 2 relates some apparently less central (when compared to supposition theory, for example) issues of Ockham's theory to relevant topics of Contemporary Philosophy, such as possible worlds, designation, demonstratives etc... In this chapter I also introduce conceptual tools which I will make use of for the reconstruction undertaken in chapter 3. Finally, Chapter 3 is an attempt to provide truth conditions for quantified, modal and tense propositions, based on the truth of singular propositions. I hereby hope to reach a rather broad account of Ockham's thinking, even though my main target is to build a coherent and correctly structured reconstruction of his theory of propositions.