§22. Perceiving Like a Roman

Concepts from jurisprudence are ubiquitous in Roman natural philosophy, and forensic judgment is the cornerstone of the Roman theory of knowledge, for which the courtroom examination of evidence is the model.

Law evidently appealed to Seneca even though he was not a trained lawyer because he wanted to do something lawyers do routinely, which is to make exact distinctions and separate evidence from counter-evidence. He needed technical concepts and technical reasoning, and found them in law. A scholar of ancient law writes of the Roman tendency to isolate the object of legal study, casting a dissecting and detached gaze upon the world, searching for invisible proportions between things and people. Stoic philosophers were like that too.¹⁷⁴

The astronomer Ptolemy and the physician Galen were contemporaries of the next century with a more ambitious stake in science and facing a skepticism grown tenaciously philosophical. They respond with detailed physical explanations of how observation works and why it is reliable, de­veloping what a scholar of Roman science describes as “fully fleshed-out empirical methodologies,” including sophisticated strategies to “control the vagaries inherent in observation through the improvement of observational techniques, through the understanding of where observational error can in­trude, and through the careful minimizing of the effects of such error”¹⁷⁵

The explanations favor vision, as they always do.

For Galen too, vision is extramissal. The emitted substance is pneuma, co­vital with blood but colorless, streaming from the eyes into the surrounding air. The pneuma flows from the soul's central power through a hollow optic nerve to the eye. On emergence from the eye it immediately combines with the adjacent air and assimilates it instantaneously, which makes the air con­tinuous with the eye, somewhat like a blind man's stick, although Galen rejects the analogy. The stick merely transmits pressure to the hand, while with the emission of vital pneuma the living power of sight “reaches out through the intervening air to the colored body” The jet of pneuma is not like a stick. It is like living tissue, like a nerve, a finger, a pseudopod momentarily vitalized by pneuma. Galen says the air “becomes for us the kind of instru­ment that the nerve in the body is at all times.” The air “becomes for the eye the same kind of instrument for the proper detection of its sense-object as the nerve is for the brain.”¹⁷⁶

Both theories work on the principle that like affects like, an idea that goes back to Empedocles in the early fifth century.

For Ptolemy, the emitted visual flux is completely passive and does not af­fect the things it encounters and illumines, though it is very sensitively af­fected by them, according to the like-by-like principle. This visual flux is like in genus (genere) with all the visual qualities—light, color, extension, and so on. Following Democritus, Aristotle, and Epicurus, Ptolemy argues that organs of sense always tell the truth about how they are affected, and are not themselves a source of error. The visual flux simply receives quali­ties simpliciter. We must therefore “pay the greatest attention of all to the simple and primary judgments that occur when the transmission of the sense presentations is uncorrupted and clear, considering them to be as infallible as human capability permits.”¹⁷⁸

In a philosophical opuscule, “On the Kriterion and the Hegemonikon’,’ Ptolemy explains the different contributions of sense perception and intellect to scientific cognition. Sense perception is the instrument by which the sub­ject under investigation is judged; intellect is the agent of judgment; logos is the means by which the agent judges.

We must neither dismiss sense perception as contributing nothing to knowledge, nor prefer it to thought. “Instead, we must allow sense perception and thought each to play its proper function, and employ each for the purpose it is naturally able to fulfill infallibly.” Sense gives infallible information about its affections, and thought judges those affections and the (hypothetical, inferred) causes, linking what is transmitted by sense with memories, and distinguishes things that have produced such affections. Anticipating what Descartes and Leibniz will say about the contribution of the senses, Ptolemy writes of thought, the inner logos, that “it submits through its own agency to reason, and discovers the proper nature of each kind of thing by successive acts of contemplation through a process of review which is continual and in­dependent of the senses.”¹⁸⁰

Galen takes a less rationalistic line. Psychic pneuma fills our nerves and is cognitively (semiotically) affected by worldly pneuma on the like-affects-like principle. The affections of worldly things pass to the eye with no loss of in­formation. “Like shakes hand with like,” he says; “like comes into communal effect with like” All of Galen's descriptions of sensory powers are couched in terms of sharing an eidos with their object.

The launching point for skeptical Pyrrhonism in antiquity is the assump­tion, ubiquitous in classical philosophy, that a claim to knowledge is a claim to philosophical knowledge of truth, which means the truth of beings, of what is, just as it is. Skeptics were inventive in confronting such claims with penetrating doubt. The reply by Ptolemy and Galen is that we can know nat­ural bodies because we too are natural bodies and respond the same as they. The method uses the body's continuity with nature to perceive nature verid- ically, courtesy of the like-affects-like law. I would not underestimate the dif­ficulty of such a theory, or suggest that Pyrrhonism is so easily answered. But that seems to be what these Roman scientists want to do, and they do it in the name of a kind of empiricism in natural philosophy.¹⁸²

After about 500, Ptolemy and Galen are mere reputations in Europe. Translations of their work from Arabic and Greek did not begin to appear until the twelfth century. Neither author had appreciable influence on Latin scientific writing during the first millennium ce. Their afterlife is under Islam, which carried on their research. It was not antiquity's culminating researches, but rather a debased Greek handbook tradition, grandly known as the Seven Liberal Arts, that determined what part of ancient science

passed to Latin readers, which was very little and degraded over time without renewal. The vitality and continuity of ancient science lies not in the West but in the East.¹⁸³