Both Ockham and Buridan had specific, nonlogical reasons for maintaining the distinct category of quality, namely, their rejection of atomism. For, in terms of the possibility of pure logical analysis, in an atomistic metaphysics, qualitative changes (such as changes of color, as opposed to quantitative changes or locomotion) could have been ‘‘analyzed away’’ analogously to the elimination of changes in the species of ‘‘shape,’’ with reference to complex connotative concepts referring to and connoting only substances and the locomotion of their quantitative parts. However, Buridan partly convinced by Aristotle’s arguments against the ancient atomists, partly seeing the troubles incurred by contemporary atomists, such as John Mirecourt and Nicholas of Autrecourt, flatly rejected atomism as ‘‘an obscure and dangerous doctrine,’’ and embraced the category of quality as containing individual entities distinct from substance and quantity (Buridan 1989:122).
Indeed, for similarly extralogical reasons, he departed even further from Ockham’s ontology. The most important of his ‘‘ontological departures’’ from Ockham can be summarized in the following points:
1. Acknowledging quantity as a category distinct from substance and quality, for purely physical reasons.
2. Positing impetus as a distinct quality to explain what we would describe as inertial phenomena.
3. Positing modes as somehow, but not really, distinct from (in the sense of being merely contingently identical with) the absolute categories of substance, quantity, and quality.
4. Positing intelligible species, but identifying them with phantasms qua the immediate objects of the agent intellect.
5. Endorsing the unicity of substantial forms, that is, denying the plurality of souls in the same individual, while distinguishing the (instrumental) powers, habits, and acts of the simple substance of the soul.
6. Positing different degrees of unity, ranging from simple substances through composite substances to merely successively persistent substances, such as rivers.
As for the first point, Buridan’s main reason for positing quantity as a distinct category, true to his Aristotelian empiricist approach to natural science, is that it seems to be necessary for the explanation of the phenomena of condensation and rarefaction in the framework of a nonatomistic, plenum theory. For in an atomistic theory, the explanation is easy in terms of the smaller or greater distance between the atoms of the body compressed or extended (which is precisely the idea in the modern kinetic theory of gases). But in a plenum theory (i. e., a theory of matter that denies the existence of a vacuum and holds that matter is a continuum), if there is no addition of matter to the substance of the thing (which is what distinguishes decompression or rarefaction from growth), then one has to say that while the substance remains the same, its quantity (i. e., its dimensions) must become greater, which is possible only if the quantity of the thing is distinct from its substance.
But similar (experimental as well as theoretical) considerations motivate Buridan’s positing a specific quality, namely, the impetus of moving bodies. His impetus is introduced, again, as a requisite explanatory principle. Given the principle of Aristotelian physics that everything that is in motion needs to be moved by a mover, phenomena that we would characterize as cases of inertial motion, such as the motion of projectiles, posed a problem: what moves, for example, an arrow shot from a bow, when it is no longer moved by the bowstring? Aristotle’s answer, namely, that it is the air still moved by the bowstring, was heavily criticized already by his sixth-century commentator, Philoponus, who favored the view that it is some impressed force imparted to the projectile by its original mover that sustains its ongoing motion. However, it was Buridan who fully worked out the idea in his theory of impetus, viewed by many historians of science as a precursor ofthe modern notion ofinertia, although in its actual description given by Buridan it is closer to the modern idea of momentum. His impetus is an impressed force (imparted to the moving body by its original mover, which brings it up to a certain speed), which is directly proportional to the speed of the moving body and to its heaviness (not quite the same as what in modern physics we would call ‘‘mass,’’ but rather a heavy body’s natural tendency to be ‘‘down,’’ at the center of the earth), and which is not spontaneously diminished, but is only weakened by the resistance of the medium. The greatest virtue of this theory is its applicability to a whole range of diverse phenomena that were either puzzling or ‘‘anomalous’’ in themselves, such as projectile motion, and/or were treated as falling under radically different explanatory models in the original Aristotelian framework, such as the acceleration of falling bodies, the motion of projectiles, the ongoing rotation of a spinning wheel, and the motion of celestial bodies, thereby providing a coherent, unitary conceptual framework for all these diverse phenomena, serving as a model for the unification of earthly and celestial mechanics in early modern science.
But aside from such explicit theoretical demands of the explanation of phenomena, Buridan, being the ‘‘relentless’’ nominalist, was always reluctant to posit really distinct entities in diverse categories; thus, in the categories other than substance, quantity, and quality, as well as in the species of ‘‘shape’’ in the category quality, he consistently applied Ockham’s eliminative strategy using nominal definitions containing absolute terms only in the ‘‘permitted’’ categories. Nevertheless, occasionally, he does not refrain from referring to what terms in the other categories connote or signify as the modifications or modes (modi) of things in the ‘‘permitted’’ categories. Thus, he would admit without further ado, for example, that while the quantity of a straight piece of wire is arranged in one way, the same quantity is arranged in a different way, if the wire is bent, that is, the same quantity has some modification at one time, and a different one at another. However, although this way of speaking involves what in modern logic we would call ‘‘quantification over’’ different modes, he would not regard this as adding any extra ontological commitment to his theory. For although the modus of the quantity of the wire at one time is different from the modus it has at another time, each is contingently identical with the same quantity at different times, thereby not adding to the number of things in the universe at any given time. This conception of modes, as being somehow different, yet without being numerically distinct, from absolute entities, was to have a bright career in later medieval and early modern philosophy, at first just undermining and eventually completely replacing the Aristotelian distinction between substance and accident. (See also the entry Substance, Accidents and Modes in this volume.)
But Buridan’s nominalist zeal also had its impact in more specific fields, such as his philosophy of the soul. Thus, although he apparently endorsed intelligible and sensible species, already diligently eliminated by earlier philosophers and theologians, such as Durand of St Pourcain or John Peter Olivi, in favor of a direct relationship between cognitive acts and their objects, he would nevertheless identify sensible species with the first receptive acts of the external senses, and intelligible species with phantasms (singular, sensory representations of singulars), insofar as they are the indirect objects of the act of the abstractive intellect forming its abstract concept, a mental act whereby it directly conceives all of the corresponding external singulars of the same kind. Indeed, his ontological economy shows up not only in his analysis of the cognitive process, but also in the analysis of the constitution of the soul and its powers or functions, as expressed in point 5 above: contrary to Ockham and the ‘‘pluralist’’ tradition in general, he argued that there is only one substantial form, one soul, in the same living individual, which alone is capable of accounting for the diverse (vegetative, sensitive, and rational) functions of the same individual (plant, animal, or human, respectively), through the diverse instrumental powers it has, as it animates the diverse organs of the same living body (or uses no organ at all, as he held, though not as a provable philosophical conclusion, concerning the intellect). But while he distinguished these ‘‘instrumental’’ powers from the soul itself, he also argued that the ‘‘principal’’ powers of the same soul (the essential abilities to carry out vegetative, sensitive, or rational functions) are nothing but the soul itself, denominated variously from its diverse operations.
Finally, we should mention Buridan’s rather original analysis of different conditions of identity and persistence through time relative to natural kinds, which apparently results in a conception that admits different degrees of unity relative to natural kinds (Buridan 1984, lb. 2, q. 7; Buridan 1509, lb. 1, q. 10). Still, this does not commit him to acknowledging anything like Aquinas’ conception of the analogy of unity and being, since for him, the conceptual order does not have to reflect the real order in such a close way as it was conceived by Aquinas (see Being).
World History
