With these reforms and a growing disenchantment with the earlier distinction between science and rhetoric, there was no longer a particular mode of discourse appropriate to scientific inquiry. The seventeenth century witnessed an increasing dissatisfaction with this breakdown of boundaries , alongside the continued erosion of the credibility of the older order. In response, efforts arose to articulate modes of truth claiming that aspired to a higher standard than humanist claims to plausibility while responding to the skepticism that worked to discredit Scholastic claims to certainty.15 At the same time, the contentious politics of the Reformation and Counter-Reformation sparked a more intense interest in rhetoric as an affective and motivational force.16 Responding to these manifold pressures and frustrations, New Scientists of the seventeenth century yet again reconceived relations between philosophy, rhetoric, and experience. Some, like Descartes, recommitted themselves to demonstrative certainty in the wake of radical skepticism, while Hobbes offered up his new science of politics focused on the passions, using mathematical reasoning and definition. In contrast to both, English empirical science, under the influence of Bacon, Boyle, and Gassendi, among others, championed experience as a source of knowledge, albeit with a more limited understanding of truth than that sought by Aristotelians or Cartesians. Locke, with his claim to experience, cut a defining figure in “the development of a family of ideas that breached the epistemological barrier between a logic and rhetoric, or knowledge and opinion.”17
This is not to say that either rhetorical or philosophical notions and practices remained unchanged by this breach. On the contrary, these new empirical scientists reworked established modes of authorizing claims to assert and legitimate their innovations in philosophy and science.18 The new emphasis on experimental natural philosophy challenged Aristotelian and Cartesian philosophy, on the one hand, and the Pyrrhonian skepticism of Montaigne, on the other. These challenges did not necessitate a radical break with these traditions, however. Instead, early empirical scientists drew from a range of traditions that they reworked for their distinctive purposes. The influence of Montaigne and other skeptics is registered in the prominent role of contingency and the persistent threat of fallible senses. Their mitigated skepticism, however, focused on modes of judgments based on experience that were registered as more or less probable. Such evidence from experience, judged by a matter of degrees rather than certain truth or falsity, challenged traditions of appealing to authoritative texts favored by both Aristotelians and humanists.19 With the new notion of probable judgment reworked from the rhetorical tradition, these experimental scientists sought authority for their new claims to knowledge, based not in tradition but in contingent experience.
This project required new methods of evaluation to distinguish matters of fact from the products of unquestioned opinion and fallible senses. In turning to experience as a source of knowledge, the English empirical scientists did not consider all experience to be equal. They developed experimental methods, but not all sources of knowledge could come from direct observation. Testimony and reports of others were also necessary, if not always as reliable. The Royal Society developed new standards and gradations of probability in order to evaluate testimony and determine what could be considered a “matter of fact” under conditions of incomplete or contested information.20 While truth through demonstration was often out of reach, a range of terms indicating degrees of uncertainty could be applied, among them highly probable, probable, mere opinion, and conjecture.21
It is important to note that probability in this era did not entail the highly formalized mathematical methods used by social scientists today, though it could involve quantification. These early empirical philosophers considered carefully, both qualitatively and quantitatively, when assent to philosophical claims was warranted. They based assent, for example, on the number and reliability of firsthand testimony and secondhand reports whose reliability might be determined by personal characteristics of observers, for example, their skill, education, and personal stakes in the outcome. These emerging scientific methods borrowed heavily from practices for evaluating the reliability of witnesses and their testimony, as established in legal practice and with roots in the rhetorical tradition.22 Where opinion had previously been excluded from the purview of science, a central question for this new natural philosophy was which opinions could be trusted as evidence of knowledge or were close enough to be useful.23
The Royal Society and its adherents were not simply taking up a new mode of scientific inquiry for a narrow group of experts. They sought new modes of reporting probabilistic claims based on experience to audiences for whom such claims were unfamiliar.24 Reporting of experimental findings was indispensable for this new scientific method and it was also essential for winning approval and support. They needed, in other words, to persuade new audiences of their experimental claims without a claim to philosophical certainty. Consequently, the new articulation of probable knowledge drawn from experience generates more than a new epistemology. To substantiate the conclusions drawn from an experiment and enable others to build upon that knowledge required that witnesses be present and their presence recorded. Experimental reports, Boyle argued, must be written so as to allow others to replicate the experiment without having been present, scrupulously recounting their methods, materials, and circumstances. Such careful performance of experiment and the recording of direct observation, however, were insufficient for the aspirations of the Royal Society, which hoped to convey their findings to a much larger public than those who might ever serve as direct observers. They sought to expand their claims of experience through vivid narratives of their experiments so they could create, in Steven Shapin’s words, “virtual witnesses.”25 Modes of writing aimed at eliciting the experience of observation, at bringing knowledge back to the senses, came to be essential for the launching of novel scientific claims through a vocabulary of probability, opinion, and experience drawn and reworked from the rhetorical tradition.
Recognizing the intersection of philosophical and rhetorical traditions and the ongoing place of rhetoric in this emerging scientific culture makes a difference for how we understand Locke’s claim to experience in the Essay. Resituating Locke within the context of early English empirical philosophy troubles the long-standing association of Locke with philosophy against rhetoric and perceptual experience against language. Douglas Casson shows how extensively this new wave of probable judgment informs An Essay Concerning Human Understanding and Locke’s body of work as a whole, including his writings on religion and politics. The Essay, in fact, comes to enact the very shift from demonstrative certainty to probability that Shapiro and others have highlighted historically. In book 4 of the Essay, Locke asserts what seems like a hard and fast distinction between knowledge and opinion, reiterating the traditional opposition between scientia and opinio. He defines knowledge as “nothing but the perception of the connexion and agreement, or disagreement and repugnancy of any of our Ideas” (4.1.1). Such knowledge, described as an immediate perception, is certain and treated as a kind of passive reception in which the understanding has no choice but to accept the proposition in question. Belief or opinion, by contrast, is qualitatively distinguished from knowledge. Belief or opinion requires an active practice of judgment of whether assent should be given to a relation of ideas “when their certain Agreement or Disagreement is not perceived but presumed to be so.” At this point in the text, Locke describes knowledge and judgment as two different faculties (4.14.4). Most propositions arise as a matter of judgment rather than certainty, and as such they are weighed in terms of probability, or likeliness of truth by degrees