It is the common opinion of philosophers that all arts and sciences discourse concerning universal things, and do not deal with singulars individually, except for the sake of example or induction. Divine nature, however, is exempt from this opinion: although it is most singular, nevertheless both divine and human philosophy dispute concerning it, because by its very singularity it contains most perfectly in itself the perfections of all things. Since, therefore, Dialectic is first in the doctrinal order of all sciences; and it alone bears the torch for all who rightly philosophize; it is necessary that it teach, as soon as possible, what the universal is, and how many are the species or forms of universals. Wherefore, after an exact prelude of first principles, in which the outlines of the whole of Dialectic are sketched, as the saying goes, with no great skill; it is fitting that those who desire to exercise the art seriously be handed, after a somewhat more accurate constitution of the art of Dialectic which teachers commonly premise before all things, a more express knowledge both of the universal itself, and of its common species — Genus, Species, Difference, Property, and Accident: which we shall attempt to do in this Philosophical Isagoge for the benefit of students of philosophy.
Another reason also accrues: Aristotle, whom all follow as leader in this art, began the explication of the whole art from the book of Categories; in which book he undertook to reduce all universal things to ten certain supreme genera, which are divided into very various species through other and other differences: and he professes to hand down certain proper attributes of them all, certain common accidents; yet he in no way declares in that treatise either what the universal is, or whether it has any species, or what force each of them has.
Finally, since their knowledge is also necessary for knowing all the genera of modes of discourse, which are posited in the reasoning of dividing, defining, demonstrating, and persuading from probabilities; no place was more suitable for this to be done than before the book of Categories.
In this little book, therefore, two things must be observed by us. One, that we neither reject all difficulties which present themselves in this matter (for it is fitting that students of philosophy already grow accustomed to handling and overcoming moderate difficulties, especially those which open easier access to what follows) nor that we put everything forward indiscriminately. For since Dialectic is a rival of first Philosophy, and moves in the same things in which that does, albeit in its own way, and seems sometimes to enter the boundaries of natural science: if we wander more freely in this place in expending the account of universals and their generation, there is danger lest we exceed the measure, and assume most things which are proper and peculiar to those sciences. Although this is not inconvenient, but rather useful, and often necessary, that one science use certain pronouncements and arguments of another with permission. For the thing of one (as someone says) asks the help of the other, and they conspire amicably. Which is all the more to be conceded to this art, since it alone is accustomed to pour back all its powers acquired from anywhere into the utility of others.
The other is, that preserving the common usage of words of the philosophers of this age, as is fitting, we derive all things from Aristotle, as far as possible, or draw them from the more common and more received opinions of the whole school; such are most of those things which are contained in the Isagoge of Porphyry. Those things, however, which he, otherwise condemned by the grave censure of the Fathers, brought in as it were proper to that little book, we, if they are true, will also use, neither as useless nor superfluous (such are almost those things which are contained in the posterior part of his little book) as if from an unjust possessor we transfer them into our use.