In the year 1328 Ockham and certain leading members of the Franciscan order became convinced that the pope of the day, Pope John XXII, was a heretic and therefore no pope. From then until his death in 1347 Ockham wrote nothing more (or almost nothing) on logic, natural philosophy, or speculative philosophy but produced a large body of books and pamphlets, commonly called his “political” writings, advocating the deposition of John XXII and his successors, Benedict XII and Clement VI. The quarrel with these popes began when John XXII issued several documents attacking doctrines and practices of the Franciscan order on the subject of poverty, doctrines the Franciscans believed to be based on the Bible and the accepted teaching of the Church. To understand Ockham's political writings, therefore, we must first consider Franciscan ideas about poverty.

Ockham's treatment of the ten Aristotelian categories plays a crucial role in his innovative nominalist program. One of his main complaints against “the moderns,” as he is wont to call his opponents, is that they treat the categories as comprising ten mutually exclusive classes of distinct entities. Indeed, the unknown author of a work written against Ockham’s logic (characteristically entitled “A very useful and realist logic of Campsall the Englishman against Ockham”), writes:

Medieval logic begins for most purposes with the work of Boethius, who attempted a Latin rendering of the standard late-antique Greek logic course. As the Boethian treatises began to be more widely studied in the ninth and tenth centuries, an indigenous Latin tradition in logic developed that reached its zenith during the twelfth century in the work of figures like Peter Abelard. It was within this indigenous tradition that the major developments in the medieval theory of inference took place and that the foundations of later pictures of meaning were laid. As the work of the Byzantine grammarian Priscian became known in Western Europe, the grammatical and semantic theory embodied in it fused with the indigenous tradition. The resulting picture was then transformed by a flood of new translations of texts of Aristotle, of Greek commentaries, and of Islamic commentaries and treatises. The thirteenth and fourteenth centuries saw the digestion of this material and its integration into the already existing tradition. Some consequences of this were new theories of the semantic properties of terms (including theories of the analysis of sentences), the development of propositional logic more or less as we know it today, the working out of the theory of the categorical syllogism (including its modal extensions), the flowering of modal logic generally, and the development of a general theory of inference. By the mid-fourteenth century these developments were more or less complete. William Ockham is one of the greatest of the figures who completed them. He is in some ways an idiosyncratic logician, but his importance can hardly be overestimated.

Pelagianism is a heresy first defined in reaction to the views of the monk Pelagius in the fifth century and attacked by Saint Augustine. In the centuries since Pelagius's defeat, theologians have repeatedly stigmatized their opponents as Pelagians. Such disputes typically pit those who emphasize predestination and original sin against those who believe in human free will and human capacity for goodness. But seldom are the arguments in these debates straightforward. Often incredibly refined, they frequently ask us to consider not the human situation as we know it but the situation before the Fall and even the options that obtained (or did not obtain) before humans were created. Not only is Pelagianism a heresy, but so is semi-Pelagianism, and so complicated are the issues involved that advocates of contrary views on relevant issues have both been accused of Pelagianism.

Standard histories have long recognized that the three most important figures in the philosophy of the High Middle Ages were Thomas Aquinas (1224/5-74), John Duns Scotus (c. 1266-1308), and William of Ockham (c. 1288-1347). Of the three, Aquinas is comparatively well known to modern readers, whereas Scotus and Ockham largely remain mere names.

Even Aquinas, however, is more foreign to students than Plato and Aristotle are, much less Descartes or Hume. Indeed, as Kretzmann and Stump have observed in The Cambridge Companion to Aquinas, such unfamiliarity is characteristic of all medieval philosophy. This sad fact is partly due to the scarcity of translations but more fundamentally to the lack of reliable modern editions of primary texts and thus of good critical analyses and studies of them in the secondary literature.

The situation does not arise from any lack of raw materials but instead, it might be argued, from just the opposite. There are many early printed editions from the late fifteenth and early sixteenth centuries and an enormous number of surviving manuscripts of medieval philosophy and theology. But the early editions are often unreliable, whereas the manuscripts frequently present wildly different versions of the same work. They are written in a highly compressed and arcane system of abbreviation, a kind of shorthand that requires special training to read; early printed editions often retain the same system. Frequently the manuscripts are incompletely cataloged or not cataloged at all, and thus their contents are discovered only by chance.

At the outset of SL, Ockham endorses Boethius's old distinction between three sorts of discourse: written, spoken, and mental. The first two, he explains, are physically perceptible, whether by the eye or by the ear, and are made up of conventional signs. The units of mental language, by contrast, are concepts. They are internal to thinking minds, and their signification is natural rather than conventional. Being mental, they are not directly perceptible - at least not in this world - to anybody but the person who internally produces them in the course of his or her private thinking. But being originally acquired as the result of a natural process, they are nevertheless strongly similar - and identically organized - from one human being to another. Although it is not a public medium of communication, mental language is potentially common to all. Mental language is prior to, and underlies, every reasonable speech utterance and provides it with meaning. Ockham's semantical theory, as presented in SL and elsewhere, is primarily an explication of the various ways in which the natural conceptual signs that constitute the language of thought are linked with their external referents; and secondarily, of the ways in which conventional discourse is derived from this mental language.

After the death of Thomas Aquinas and well into the fourteenth century, medieval philosophers strenuously debated the nature of human cognition and the means by which it is achieved. Ockham took a radical stand in that debate, rejecting the widely accepted view at the heart of Aquinas's account, namely, that cognition is mediated by intelligible and sensible species. (I will explain this technical use of 'species' in Section I.) In a variety of texts, Ockham went to great lengths to argue against not only what many scholars take to be Aquinas's own account but also a revised version of it adopted by Duns Scotus. Why did Ockham take such a stand? Why did he insist on rejecting species as a means of cognition, and what alternative theory did he adopt?

The full and complete answer to these questions would no doubt require a very lengthy book. I want to give only one part of the answer, focusing on Ockham’s theory of the cognition of an individual extramental material object. Even so, for the sake of brevity, I will leave to one side an array of topics that are to some degree relevant. Thus, for example, I will leave largely unexamined Ockham’s response to the optics of his time, which relied on species, and his reinterpretation of certain texts of Aristotle’s, which seem to give species a prominent place. Instead I will focus just on Ockham’s account of an ordinary and comparatively simple cognitive experience. I lift my eyes, look across the room, and see a coffee cup. What is it about me that explains my cognition of the cup for Ockham? And why does he feel impelled to reject Aquinas’s view of the cognitive processes involved in perception of this sort?

William of Ockham has long been considered one of the foremost figures in the history of medieval philosophy and theology. As such his thought is often contrasted with that of the other seminal thinkers of High Scholasticism: Thomas Aquinas, Henry of Ghent, Giles of Rome, and John Duns Scotus, as if those were the appropriate and sufficient voices of debate within which Ockham's thought was developed. The completion of the critical edition of Ockham's philosophical and theological writings has, on one level, confirmed that picture and revealed Scotus as the single most important figure on Ockham's intellectual horizon. The editors, however, along with scholars working on lesser-known figures in the early fourteenth century, have at the same time uncovered a more complex picture of intellectual exchange in which Ockham's immediate contemporaries - those active between 1305 and 1325 - exercised a profound impact on his thought, and he on theirs.

Other contributions of recent scholarship that change or at least refine the way Ockham is viewed today are a more extensive knowledge of the lives of those with whom he interacted, the educational system of the Franciscan order that determined the physical settings in which Ockham was active, and the structure and intellectual activity at universities and other studia in England and on the Continent. These allow a fresh examination – a more nuanced picture – of Ockham’s intellectual heritage and the influence his thought had on subsequent generations.

The first thing one learns about William of Ockham's philosophy is usually that he was a “nominalist.” But sometimes it is not explained just what Ockham's nominalism was. For medieval nominalism, like its modern namesake, took many sometimes surprising forms.

At least two distinct themes in Ockham’s metaphysics have been called nominalism: (1) his rejection of universals and their accoutrements, like the Scotist formal distinction, and (2) his program of what can be called “ontological reduction,” namely his eliminating many other kinds of putative entities, whether universal or not, and in particular his cutting the list of real ontological categories from Aristotle’s ten to two: substance and quality (plus a few specimens of relation in theological contexts). Although I will say something about both themes in this chapter, the emphasis will be on the second.

These two themes are independent of one another. One might deny the reality of universals, as Ockham did, yet maintain that individual entities are needed in more or fewer categories. Thus, John Buridan rejected universals as resolutely as Ockham ever did but thought there are real, irreducible entities in the category of quantity, which Ockham denied, as well as in the categories of substance and quality. Conversely, one might think some of the categories in Aristotle’s list can be reduced to others while insisting that universals, not just individuals, are needed in some or all the remaining categories. Thus, Ockham’s contemporary Walter Burley thought the categories could be reduced to the same list of three Buridan allowed, but he was a realist about universals.

When Galileo Galilei succeeded in transforming physics into a quantitative, mathematical science, his effort was the culmination of a tradition that we can trace to the Greeks. At the same time, the success of Galileo's effort represents the end of the fundamentally qualitative approach to nature that is characteristic of Aristotelian natural philosophy. Aristotle's philosophy of nature comprises all of the texts related to the study of nature, the metaphysical principles of nature, change and motion, earth, the heavens, his studies in biology, and the shorter treatises on the senses and perception, all culminating in his major work on the soul.

In the Middle Ages the interpretation of Aristotle’s natural philosophy continued the classical tradition of interpretation in the context of Peripatetic, Neoplatonic, Stoic, Christian, Islamic, and other agendas. Although there were scholars who contributed to the exact sciences, in texts that fit in the tradition of medieval natural philosophy we find for the most part philosophical discussions of texts, typically of Peter Lombard’s Sentences, Aristotle’s Physics, and of other treatises by Aristotle as well as of the commentaries of Averroes. William of Ockham wrote two very long texts on natural philosophy, two shorter accounts, and numerous comments dealing with questions on natural philosophy in his massive Commentary on the Sentences. Yet, even when he shows himself to have some familiarity with a specific discipline (as in theories of vision), his use of it tends to be sketchy and highly selective.

In 1943, Philotheus Boehner, the distinguished Ockham scholar, published an article entitled “The Notitia Intuitiva of Non-existents according to William Ockham,” an article that has been extremely influential ever since.

This date can be regarded as a turning point in the history of the misunderstanding, in whole or in part, of Ockham’s theory of intuitive and abstractive cognition in this century. Before that date, eminent scholars2 had ascribed to Ockham the very doctrine of intuitive cognition of nonexistents he had rejected. In his article, Boehner proceeded to denounce their mistake. Ironically, however, in the very same article, he introduced a new mistake, now bearing on the whole of the theory. Unaware that he had misread a certain text, he ascribed to Ockham a theory of intuitive and abstractive cognition that neither he, nor anyone else as far as I know, ever subscribed to. Though this mistake was introduced in the literature more than fifty years ago, not only has it gone undetected, it has been repeated by most subsequent scholars who have written on the subject, all of whom have studied Boehner’s article. As a result, the mistake has become completely entrenched, and many scholarly discussionson Ockham’s epistemology have been based on the assumption that Ockham held a certain doctrine which, in fact, he never subscribed to, nor even conceived of.

Among other things, Ockham is notorious for his doctrine of the liberty of indifference: the notion that created willpower is power to will, to nill, or to do nothing with respect to any object. By contrast with his great medieval predecessors, many estimate, Ockham has staked out a position fraught with disadvantages. First, it cuts will off from nature. The liberty of indifference turns created wills into neutral potencies unshaped by natural inclinations. Second, it “frees” will from reason's rule: no matter what reason dictates, created willpower can disobey.

To some, such consequences have seemed momentous for ethics because they are inconsistent with a kind of naturalism. For example, Maurer writes that

Analytic philosophers specializing in medieval philosophy have tended to focus on those aspects of Catholic medieval thought that seem relevant to research programs already firmly established within the mainstream of contemporary academic philosophy. In this way they have tried to convince other philosophers that the Catholic medieval thinkers, despite their theological presuppositions, have something useful to contribute to current discussions. The tendency in question has been especially pronounced in the case of William of Ockham because he is at his best when doing ontology and philosophical semantics, two areas that have figured prominently in recent analytic philosophy and that seem safely removed from distinctively Catholic beliefs.

Undeniably, much valuable reflection on Catholic medieval thought has been generated by this desire to show how certain parts of the works of Ockham and the others might bear on contemporary problematics or even inspire us to reconfigure those problematics; indeed, many academic philosophers who would not otherwise have noticed the medievals have thereby been led to treat them as fullfledged interlocutors. Still, to limit ourselves to this fragmentary approach prevents us from understanding these thinkers as they understood themselves and renders us vulnerable to the abiding temptation to refashion their work so as to make it suit our own cultural and philosophical biases.

William of Ockham's semantic theory was founded on the idea that thought takes place in a language not unlike the languages in which spoken and written communication occur. This mental language was held to have several features in common with everyday languages. For example, mental language has simple terms, not unlike words, out of which complex expressions can be constructed. As with words, each of these terms has some meaning, or signification; in fact Ockham held that the signification of everyday words derives precisely from the signification of mental terms. Furthermore, the meaning of a mental expression depends directly on the meaning of its constituent terms, as is the case with expressions in more familiar languages.

As one might expect, there are many important differences between mental language and everyday languages. For example, mental languages signify their objects naturally rather than conventionally. At a more concrete level, Ockham suggested that numerous features of spoken or written language – participles and pronouns, for example – might not exist in mental language.

INTRODUCTION TO THE PROBLEM

The term ‘Jewish Christianity’, in German ‘Judenchristentum’, was current in the eighteenth century, but was brought to prominence by Ferdinand Christian Baur. He used it to describe what he took to be an important phenomenon within the Christianity of the first two centuries ce. On this at least most scholars can agree. But here perhaps agreement can be said to cease, for, in spite of a history of investigation stretching back to the early 1830s, many questions relating to Jewish Christianity, its history, origins and social and religious profile, remain matters of controversy.

There are a number of reasons for the confused state of scholarship on this question. First, insofar as we know, no one in the ancient Church or synagogue referred to themselves, or were referred to, as Jewish Christians. This gives rise to a number of problems, not least that of defining the term. Secondly, we have to contend with the inadequacy of the relevant primary sources. These are few in number, and nearly all written by those who were opposed to Jewish Christians, and had an incomplete and/or confused knowledge of what Jewish Christians might have thought. Those apparently written by Jewish Christians are often preserved in fragmentary form (this particularly applies to the Jewish Christian Gospels), and in complex corpora like the Pseudo-Clementine Homilies and Recognitions, which present literary-critical problems of an almost insurmountable kind. Moreover, while we may be in a position to know what some Christians thought about Jewish Christians and how they described them, we appear to be much less well informed about what Jews thought about Jewish Christians.