Abelard follows Guillaume de Champeaux, with whom he was for the earlier part of his career a contemporary. Confounding, as it would seem, ideas with universals, and universals with abstractions, he denied alike Rosceline's doctrine that they are mere words, and Guillaume de Champeaux's doctrine that they are entities or existences a parte rei, and maintained that they are conceptions, really existing in mente, but not in re. Hence his philosophy is called Conceptualism. He would seem to have held that universals are formed by the mind operating on the concrete objects presented by experience, not, as since maintained by Kant, that they are necessary forms of the understanding. Thus, humanitas, humanity, is formed by the mind from the concrete man, or homo. There is no humanity in re; there are only individual men. In the word humanity the mind expresses the qualities which it observes to be common to all men, without paying attention to any particular man. The idea humanity, then, is simply the abstraction or generalization of these qualities. Abelard, it would appear from this, makes what we call the race a property or quality of individuals, which, of course, excludes the idea of generation. There is, as far as we can see, no essential difference between the conceptualism of Abelard and the nominalism of Rosceline; for, by denying the existence in re of genera and species, and making them only conceptions, it recognizes as really existing only individuals or particulars.
St. Thomas Aquinas, than whom no higher authority in philosophy can be named, and from whose conclusions few who understand them will be disposed to dissent, differs from each of these schools, and maintains that universals are conceptions existing in mente cum fundamento in re, or conceptions with a basis in reality, which is true of all abstractions; for the mind can form no conceptions except from objects presented by experience. I could form no conception of whiteness if I had no experience of white things, or of roundness if I had seen nothing round. I imagine a golden mountain, but only on condition that gold and mountain are to me objects of experience. This is certain, and accords with the peripatetic maxim, Nihil est in intellectu, quod prius non fuerit in sensu, which Leibnitz would amend by adding, nisi ipse intellectus, an amendment which, perhaps, contains in germ the whole Kantian philosophy.
But St. Thomas, as we shall see further on, does not confound ideal with universals, nor does he hold genera and species to be simply the abstraction or generalization of the qualities of individuals or particulars. Genera and species are real, or there could be no generation. But the genus or species does not exist apart from its individualization, or as a separate entity. There are no individuals without the race, and no race without individuals. Thus the whole race was individualized in Adam, so that in his sin all men sinned. But as genera and species, the only real universals, do not exist apart from their particulars, and are distinctly possessed or apprehended only as disengaged from their particulars, which is done only by a mental operation, St. Thomas might say they exist in mente cum fundamento in re, without asserting them to be real only as properties or qualities of particulars.
Plato is commonly held to be the father of the ideal philosophy or ideal realism. We know very little of the philosophy that prevailed before him, and cannot say how much of the Platonic philosophy is original with him, or how much of it he took from his predecessors, but he is its originator as far as our knowledge extends. It is from him that we have the word idea, and his whole philosophy is said to be in his doctrine of ideas; but what his doctrine of ideas really was is a question. He seems when treating the question, What is it necessary to know in order to have real science? to understand by idea causa essentialis, or the thing itself, or what in anything is real, stable, and permanent, in distinction from the sensible, the phenomenal, the variable, and the transitory. The real existence of things is their ideas, and ideas are in the Logos or divine mind. These ideas God impresses on an eternally existing matter, as the seal upon wax, and so impressed they constitute particulars. Aristotle accuses Plato of placing the ideas extra Deum, and making them objects of the divine contemplation, but the accusation is not easily sustained; and we think all that Plato does is to represent the ideas as extra Deum only as the idea or design of a picture or a temple in the mind of the artist is distinguishable from the artist himself. But in God all ideas must be eternal, and therefore really his essence, as is maintained by St. Thomas. If this is really Plato's doctrine, it is dualism inasmuch as it asserts the eternity of matter, and pantheism inasmuch as the ideas, the reality of things, are identical with the divine mind, and therefore with God himself. On this doctrine, what is that soul the immortality of which Plato so strenuously maintains? Is it the divine idea, or the copy of the idea on matter?
When treating the question, How we know? Plato seems to understand by ideas not the ideas in the divine mind, but their copies impressed on matter, as the seal on wax. According to him, all knowing is by similitude, and as the idea leaves its exact image or form on matter, so by studying that image or copy we arrive at an exact knowledge of the idea or archetype in the divine mind. This is plain enough; but who are we who study and know? Are we the archetypal idea, or are we its image or copy impressed on matter? Here is the difficulty we find in understanding Plato's doctrine of ideas. According to him all reality is in the idea, and what is not idea is phenomenal, unsubstantial, variable, and evanescent. The impress or copy on matter is not the idea itself, and is no more the thing itself than the reflection I see in a mirror is myself. Plato speaks of the soul as imprisoned in matter, and ascribes all evil to the intractableness of matter. Hence he originates or justifies that false asceticism which treats matter as impure or unclean, and makes the proper discipline of the soul consist in despising and maltreating the body, and in seeking deliverance from it, as if our bodies were not destined to rise again, and, reunited to the soul, to live forever. The real source of Manichaeism is in the Platonic philosophy. We confess that we are not able to make out from Plato a complete, coherent, and self-consistent doctrine of ideas. St. Thomas corrects Plato, and makes ideas the archetypes, exemplars, or models in the divine mind, and identical with the essence of God, after which God creates or may create existences. He holds the idea, as idea, to be causa exemplaris, not causa essentialis, and thus escapes both pantheism and dualism, and all tendency to either.
Aristotle, a much more systematic genius, and, in my judgment, a much profounder philosopher than Plato, rejects Plato's doctrine of ideas, and substitutes for them substantial forms, which in his philosophy mean real existences distinct from God; and which are not merely phenomenal, like Plato's copies on wax. True, he, as Plato, recognizes an eternal matter, and makes all existences consist of matter and form. But the matter is purely passive; and, as nothing, according to his philosophy, exists, save in so far as active, it is really nothing, exists only in potentia ad formam, and can only mean the ability of God to place existences after the models eternal in his own mind. His philosophy is, at any rate, more easily reconciled with Christian theology than is Plato's.
Yet Aristotle and the schoolmen after him adopt Plato's doctrine that we know by similitude, or by ideas in the sense of images, or representations, interposed between the mind and the object, or thing existing a parte rei. They suppose these images, or intelligible species, form a sort of intermediary world, called the mundus logicus, distinguished from the mundus physicus, or real world, which they are not, but which they image or represent to the understanding. Hence the categories or praedicaments are neither forms of the subject nor forms of the object, but the forms or laws of logic or this intermediary world. Hence has arisen the question whether our knowledge has any objective validity, that is, whether there is any objective reality that responds to the idea. Perhaps it is in this doctrine, misunderstood, that we are to seek the origin of scepticism, which always originates in the speculations of philosophers, never in the plain sense of the people, who never want, when they know, any proof that they know.
This Platonic and peripatetic doctrine, that ideas are not the reality, but, as Locke says, that "with which the understanding is immediately conversant," has been vigorously assailed by the Scottish school, which denies intermediary ideas, and maintains that we perceive directly and immediately things themselves. Still the old doctrine obtains to a very considerable extent, and respectable schools teach that ideas, if not precisely images, are nevertheless representative, and that the idea is the first object of mental apprehension. Balmes never treats ideas as the object existing in re, but as its representation to the mind. Hence the importance attached to the question of certainty, or the objective validity of our knowledge, around which Balmes says turn all the questions of philosophy; that is, the great labor of philosophers is to prove that in knowing we know something, or that to know is to know. This is really the pons asinorum of modern philosophy as it was of ancient philosophy: How know I that knowing is knowing, or that in knowing I know? The question as asked is unanswerable and absurd, for I have only to know with which to prove that I know, and he who knows knows that he knows. I know that I know says no more than I know.
The quarrel has arisen from confounding ideas, universals, genera and species, and abstractions or generalizations, and treating them all as if pertaining to the same category. These three things are different, and cannot be scientifically treated as if they were the same; yet nominalists, realists, and conceptualists recognize no differences among them, nor do the Platonists. These hold all the essential qualities, properties, or attributes of things to be ideas, objective and real. Hippias visits Athens, and proposes during his stay in the city to give the eager Athenians a discourse, or, as they say nowadays, a lecture, on beautiful things. Socrates is delighted to hear it, and assures Hippias that he will be one of his audience; but as he is slow of understanding, and has a friend who will be sure to question him very closely, he begs Hippias to answer beforehand a few of the questions this friend is certain to ask. Hippias consents. You propose to discourse on beautiful things, but tell me, if you please, what are beautiful things? Hippias mentions several things, and finally answers, a handsome girl. But that is not what my friend wants to know. Tell me, by what are beautiful things beautiful? Hippias does not quite understand. Socrates explains. All just things, are they not just by participation of justice? Agreed. And all wise things by participation of wisdom? It cannot be denied. And all beautiful things by participation of beauty? So it seems. Now tell me, dear Hippias, what is beauty, that which is so not by participation but in itself, and by participation of which all beautiful things are beautiful? Hippias, of course, is puzzled, and neither he nor Socrates answers the question.