Aquinas was an empiricist. Therefore, he thinks that philosophers must begin with “things and, in the course of their speculations, they explain knowledge in terms of what they know about the being of the things that are.” In his Commentary on Aristotle’s Physics Aquinas states, approvingly, “But those things are better known in themselves which have more being, because each thing is knowable insofar as it is being. However, those beings are greater which are greater in act. Whence these are the most knowable by nature. For us, however, the converse is true because we proceed in understanding from potency to act. Our knowledge begins from sensible things which are material and intelligible in potency.”
In order to find out what knowledge is, and how we obtain it, we must begin with the thing being known, with being, because knowledge is always of something, and the way in which knowledge is gained varies, depending on the thing known. When we attempt to make knowledge into a vague criterion by which we can judge knowledge of everything, we lose our capacity to have knowledge of anything, except concerning the inner workings of our own minds.
First we need to look at the existent (for the key terms see part 3), find out what it is, and then once we know what it is, we can then examine our knowledge and find out what knowledge of that existent, looks like, how we got it, and how we can get it again. This is how bodies of knowledge, such as physics, biology, chemistry, mathematic and metaphysics are formed. It is perhaps best to illustrate this point by looking at a concrete example in real life. Serge Pronovost illustrates this point as follows:
Before I leave to go fishing, I do not say to myself: ‘Well, before I begin fishing I want to be sure that I will have success, I will, without having ever fished, establish a fishing method that is infallible.’ This is not how primitive men proceeded. First, he saw that fish exist (the object of fishing) and seeing as he was hungry, he tried to catch them with the only natural equipment that were available to him, his hands, his eyes, etc. Confronted by failure, and partial success, and desiring to get better results, he reflected about the actual act of fishing, beginning with his experience of the object, and trying to create equipment that would complement his natural equipment. He must have seen that even though all of the different species of the object (all fish) necessitate similar equipment (a method) because they are all fish, certain species necessitate unique equipment (a method). The shark is not a trout and necessitates a distinct method, even though it is, like the trout, a fish. In this way, with time, the art of fishing developed into an epistemology of fishing: ‘Can we really catch fish and how do we do it?’ There are two parts to this art, or method: a common part, as these are all fish, and a specific part, as there are numerous different species of fish and there are different ways of getting or catching them. Therefore, the art flows out of experience. What I am saying here about fishing can be said of all the arts (painting, sculpture, architecture, cooking, etc.). It is the same with the art of reasoning. Man started reasoning ‘naturally’ about facts of experience (life, death, beauty, good, love). And, running into difficulties and obstacles, and desiring to come to a more perfect knowledge, he reflected about the act of thinking itself just as he who wishes to have the best results in fishing must think about the act itself of fishing. From this ‘second’ reflection he discovered a second set of equipment (types of definitions and types of reasoning, all the equipment that constitute logic) which complement the first equipment which is natural reasoning. Some of the equipment that he discovered are common to all truths (the common mode of knowing) and other are specific to certain objects of knowledge (the mode that is proper to each type of science).
This is the way that we will gain knowledge of man and his capacity for knowledge. Each object of knowledge is known in a different way. As Gilson notes, concerning the two sciences of psychology and ontology, “The two sciences must use different methods because their objects are different.” You cannot use the same methods that you use in one science, in all of the sciences. This is the temptation that, when given into, causes so much trouble in philosophy. Further on, Gilson outlines the limits of some of the sciences in explaining sensible reality, then, after briefly discussing psychology, he concludes that, “There is no end to such a breaking up of the inner unity of the self. It is no wonder, then, that science considers the inner life to be made up of distinct elements subjected to necessary laws, just as if the nature of the soul were the same as that of physical reality. But the continuity of consciousness and the very possibility of liberty perish in the process.” We do not come to knowledge of physics in the same way that we come to knowledge of biological things like the heart.
In the same way, we cannot begin with knowledge of what God is, and then use that knowledge to prove God’s existence. We must prove that God exists before we can even attempt to talk about God. At this point, however, the example breaks down, because, for Aquinas, we cannot, in this life, know what God is. In this life, we can only gain “knowledge” of God through his effects. As such, we can know that he exists, but anything that we say about God will be inferred from his effects, and applied to God by analogy.
Frederick D. Wilhelmsen, Man’s Knowledge of Reality: An Introduction to Thomistic Epistemology (Englewood Cliffs, NJ: Prentice-Hall, 1956), 18.
Thomas Aquinas, Commentary on Aristotle's Physics, 2nd ed., trans. Richard J. Blackwell, Richard J. Spath, and W. Edmund Thirlkel (Notre Dame, IN: Dumb Ox Books, 1995), 4.
Serge Pronovost, Email message to author, March 14, 2011. This quotation was originally composed in French as follows: “Avant d’aller à la pêche, je ne me dis pas: ‘Bon, pour être certain d’avoir du succès avant même de commencer à pêcher, je veux établir une méthode de pêche infaillible indépendamment de l’expérience.’ Ce n’est pas ainsi que l’homme primitif a procédé. Il a d’abord vu qu’il existe des poissons (l’objet de la pêche) et comme il avait faim, il a dû essayer des les attrapper au moyen de ses instruments naturels que sont les mains, les yeux, etc. Puis, confronté à des échecs et des succès partiels et désirant parvenir à de meilleurs résultats, il a réfléchi à l’acte même de la pêche en prenant pour point de départ son expérience de l’objet, et essayant de créer des instruments qui complèteraient ses instruments naturels. Il a dû voir que même si toutes les espèces relatives à cet objet (tous les poissons) nécessitaient des instruments (une méthode) communs car ce sont tous des poissons, certaines espèces exigeaient des instruments (une méthode) spécifiques. Le requin n’est pas une truite et exige une méthode distincte même s’il est un poisson comme la truite. C’est ainsi qu’avec le temps s’est développé l’art de la pêche, une épistémologie de la pêche : ‘peut-on vraiment attraper du poisson et comment y arriver?’ Dans cet art ou cette méthode il y a un double volet : une partie commune, car ce sont tous des poissons, et une partie spécifique, car ce sont des espèces différentes et il existe donc des manières différentes de les ‘saisir’ ou de les ‘prendre’. Donc, l’art découle de l’expérience. Ce que je dis ici de la pêche se dit de tous les arts (peinture, sculpture, architecture, art culinaire, etc.) Il en est de même pour l’art de raisonner. L’homme a commencé par raisonner ‘naturellement’ sur des données d’expérience (la vie, la mort, la beauté, la bien, l’amour). Puis, rencontrant des difficultés et des obstacles et désirant parvenir à une connaissance plus parfaite, il a réfléchi à l’acte même de réfléchir tout comme celui qui désirait avoir de meilleurs résultats à la pêche a dû réfléchir à l’acte même de pêcher. À partir de cette réflexion ‘seconde’ il a découvert des instruments seconds (les sortes de définitions et les sortes de raisonnements, tous ces instruments qui constituent la logique) qui complétaient cet instrument premier qu’est la raison naturelle et dont certains sont communs à toutes les vérités (le mode commun de connaître) et d’autres propres à certaines catégories d’objets de connaissance (le mode propre à chaque sorte de science).” [Translation is mine.]
Etienne Gilson, Thomas Langan, and Armand Maurer, Recent Philosophy: Hegel to the Present (1966; repr., Eugene, OR: Wipf and Stock Publishers, 2005), 1:252.