Fr. William A. Wallace, O.P., The Modeling of Nature: Philosophy of Science and Philosophy of Nature in Synthesis (Washington, DC: The Catholic University of America Press, 1996), 263-264.

Types of Knowing. Aristotle’s term for knowledge is episteme, and by this he intends a type of perfect knowing that may be expressed in certain and necessary conclusions. Norms for attaining this type of knowledge are set out in the Posterior Analytics [...] The Greek term episteme translates into Latin as scientia, science in the strict sense, which attains truth with certitude and sets the ideal toward which scientists aspire even in the present day. Admittedly the ideal is difficult to achieve, and throughout history investigators of the world of nature have had to content themselves with a lesser goal—knowledge that is probable, though not absolutely certain. The Greek term for this type of knowing is doxa, and it translates into Latin as opinio, usually rendered into English as opinion. To have opinion in this sense is to assent to a proposition rather than to its opposite, but with an awareness that the proposition might be false and the opposite true. Norms for attaining probable knowledge of this type, also referred to as dialectical or verisimilar knowledge, are explained in Aristotle’s Topics. Yet a third type of knowing is even less firm than opinion; this occurs when a person cannot decide between two contradictory propositions and yet inclines to accepting one over the other. The Greek term for this type of knowing is pistis, and it translates into Latin as fides or persuasio. The English equivalent would be belief or persuasion. Norms for inducing this type of assent are worked out in Aristotle’s Rhetoric. Whereas the Topics establishes norms for discerning probabilities based on logic, the Rhetoric takes into account ways in which people are additionally persuaded by appeals to the emotions and to the authority or character of the one persuading. 

In the present chapter the focus is on science as probable reasoning, leaving the possibility of science as demonstrative or epistemic for consideration in the following chapter. As already noted, the consensus concerning recent science is that its results are always provisional or fallible and therefore always subject to revision. On this understanding, within an Aristotelian context modern science would be considered a species of opinion, highly probable opinion perhaps, but opinion nonetheless. Whether this judgment would apply to the entire content of science or only to the great majority of its conclusions need not be addressed here, since it will be considered in the next chapter. During most of modern science’s history up to the end of the nineteenth century, however, scientists generally subscribed to the traditional notion of science as certain and necessary knowledge. The fallibilist view seems to be a twentieth century development, prompted largely by advances in physics associated with quantum theory and the theories of relativity. Toward the end of the twentieth century, moreover, the situation has been further exacerbated by the proposal that science itself is to be identified with rhetoric. Rather than say that science is opinion, some would maintain that it is essentially belief. They may add the qualifiers “justified” and “true” to the “belief,” but considering the special meanings that attach to these qualifiers this does not substantially alter the fiduciary character thus attributed to the scientific enterprise. And, although the “rhetorical” aspect of modern science was initially prompted by the way scientists package and present their results in the late twentieth century, the thesis has been enlarged to apply to the entire history of science. On this view, some would hold that all of science is said to be “socially constructed,” facts no less than theories, and for them persuasion comes to be regarded as the principal vehicle for science’s acceptance.

Within an Aristotelian context, as has been said, modern science would be regarded as a species of opinion, namely, highly probable opinion. It should be pointed out, therefore, that for Aristotle opinion (doxa) is not a monolithic notion but one that permits of degrees. The highest degree of opinion, and the one that most closely resembles truth in its verisimilitude, is what Aristotle calls endoxa, a term usually translated as reputable opinion or expert opinion. In many texts Aristotle accords endoxa a factual status, equating them with phainomena or perceived appearances and so putting them on a par with empirical data. Similarly he expands his notion of truth to include truths that are partial and obscure, and points to such truths as those that are contained in, or attendable from, endoxa. Aristotle also insists that probable or dialectical reasoning of the type he explains in the Topics is necessary for establishing the principles on which epistemic knowledge is based. Thus he allows for a transition from probable knowledge to certain knowledge, from dialectics to demonstration, as one gains progressively truer and clearer knowledge of a particular subject matter.

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