I just finished reading "Presuppositions and Realism" by Linus J. Thro, from "An Étienne Gilson Tribute", p. 309-325. It was was quite interesting. Here is a quick resumé of his article, and some thoughts that it inspired.
He argues that Realism (both Metaphysical and Epistemological) is without presuppositions. He begins by discussing different views on presuppositions (Hegel, Kierkegaard, Merleau-Ponty, and Nagel). He then explains some of the terms used by authors who discuss presuppositions (commitment, pre-philosophical influences, and philosophical presuppositions). He notes that it is important to make a distinction between "a presuppositionless beginning in philosophy and actually to begin without a presupposition. (p. 316.)" Finally he sets out his argument.
His basic contention is "that realism does not assume, presuppose, or dogmatically assert that there are knowable things existent and available to knowledge, and that man operating normally has true knowledge about them. On the contrary, I contend that both are discovered in ordinary human experience, and that realism, in reflective evaluation of the self-evidence of these insights into the real, stands upon them as its basic principle. The evidence is accepted because it is understood to be unquestionable. Hence, realism, I maintain, is founded, not upon an unexamined and unjustified presupposition of a harmony between the 'subjective logos' and the 'objective logos' but upon the discovery in experience of the intelligibility of being and upon the development of its own directive principle as realism through reflection upon the evidentiality of being. (p. 317-318)" The central point of Realism, Thro says, is "the cognitive immediacy of the evidence of existence. (p. 381)" Knowledge just is the actual union of the knower with the thing known. This is a description of the event of presuppositionless knowing. There just is no presupposition "that it is possible or impossible to know X". The knower and the doubter alike, the naturalist and the theist alike, insomuch as they are, are confronted by beings. The being-appeared-to-by-X may be interpreted differently by different people, but it is a presuppositionless state of affairs that is common to all beings that are capable of presuppositions.
The author gives a couple of illustrations from scientific research. He concludes by noting that "realism, as I understand it, rests its case for a sound starting-point upon the twin elements which reflection finds in ordinary knowledge. There is the givenness of the subjective experiencing, but only simultaneously with the inescapable givenness of what is experienced; the experiencing is not a detached phenomenon any more than the experienced; neither is accounted for in terms of impressions, ideas, representations, or sense-data, whether atomic or contextual. What is experienced may be a remembered event, or a present pain, or, as the realist will insist is the most usual situation, some other reality apart from the experiencer but acting in some way upon him. (p. 324-325)" Thro concludes that if his description of Realism is accurate, if it is an accurate description of human knowledge, "then realism cannot be charged with resting upon the unjustified assumption that beings exist and are understood in experience. On the contrary, these are not presuppositions but are the fundamental insights in human experience upon which all human knowledge, action, and aspiration depend. (p. 325)"
If such is the case, then although different people may interpret (give sense or meaning to) different beings that present themselves to those people (those different beings may be beings "of reason", or "real" beings), those interpretations can be contested, discussed, refuted or defended. Just because one person has an interpretation of some being that is present to them, that does not mean that their interpretation is true. Interpretations may be true or false. However, to even be able to talk about true or false interpretations implies the possibility of being able to test them for truth. For a realist, in order to test the truth or error of one's judgements about the beings that are presenting themselves and impressing themselves upon the knower, the knower must "return" to the being that is presenting itself, either through the senses, or through reflection, and allow oneself to be presented to again, and again. Some have said that there is no way to return to the very same being through the senses, but in a sense there is. That is, in order to even be able to say "this X was that Y", one must be able to identify something about both X and Y that is common to both. For two different artifacts (i.e. - a bed made out of what was once planks of wood), or a substance that came to be from another substance (i.e. - the corpse of a formerly living animal), it may be common matter. For an X that is still an X, but has changed in some accidental manner (i.e. - place, size, colour, etc.) there appears to be a sort of X-ness that is common to the former and the latter X which allows one to say, this is an X. In this latter case, it is possible, in a sense to constantly return to the same X and to allow oneself to be appeared to by this X, so as to verify one's original judgments about X. This, in fact, what one might call the principle of the continuity or the uniformity of Nature (cf. Thro, 315.), is the primary presupposition of science. For the realist, the principle of the uniformity of nature is not a presupposition, but a deduction based upon observation. This principle, which is necessary even for the possibility for Christian theology (one cannot possibly think that the Bible that one is currently studying is the same as the Bible that one studied yesterday, unless the principle of the uniformity of nature is true), underlies all of the sciences. It is based upon the notion that things have "natures" (that which is common to X1 and X2, which allows one to say that they are both X), and that those natures remain the same throughout change. For the realist, this notion is a deduction from being-presented-to, and reflection on this being-presented-to. As such it is not a presupposition for the realist, but a discovery.
If all that "is" has a nature (even beings "of reason"), and it is that nature that presents itself to the knower when a being presents itself to the knower, and knowledge just is the intellectual union of that nature with the knower, then it is possible to know the nature of those things which are and which present themselves to the knower. If it is possible to know the nature of those things which are and which present themselves to the knower, then it is possible to discover truth and error by continually reflecting on the nature of that which presents itself to the knower. In this way, through hard work and a persistent desire to know truth, knowers will be able to arrive at some knowledge of truth.