DOI: 10.4324/9780415249126-A049-1
Version: v1,  Published online: 1998
Retrieved May 18, 2022, from

6. The criteria of truth

Since, then, Epicurus’ version of atomism allows the reality of sensible properties, he does not inherit Democritus’ motive for casting doubt on the veracity of sense–perception (see Democritus §3). And indeed for Epicurus the primary ‘criterion of truth’ is the senses: ‘All sensations are true.’

The soul is an atomic structure spread throughout the body, but with a command centre which houses rational thought (these can be considered functionally, although not anatomically, equivalent to the nervous system and brain, respectively). A sensation is the soul’s mechanical but conscious registering of the phenomenal properties of an external body. In the paradigm case of sight, this occurs because the outer layers of bodies are constantly streaming off them in all directions, taking the form of ‘images’ (Greek eidōla, Latin simulacra), atom-thin films of matter which more or less preserve their colour and shape in transit (Letter to Herodotus 49–52,Lucretius IV 26–268). Streams of these can enter the eye, producing vision; isolated ones can also directly enter the mind, asleep or awake, and enable it to visualize objects (Lucretius IV 722–822, 962–1036). Importantly, any act of picturing, by the eyes or the mind, is the registration of one or more images arriving from outside. The infallibility of the senses consists in their mechanical registering of these images, without adding, subtracting, embroidering or interpreting. A useful analogy is photography, which ‘cannot lie’ because it merely records mechanically the patterns of light arriving at the camera lens, and leaves it to us to interpret what they represent. All sensations are bona fide evidence about the external world. All error lies in the ‘added opinion’ (to prosdoxazomenon) by which the mind interprets these data. Note that truth here is representational, not propositional: the sensation is true because it accurately represents the physical data reaching the sense organ. The opinion based on it may be regarded as true or false according to whether or not it succeeds in representing accurately the state of affairs which caused the sensation. Even in a case of outright delusion it is the mind which, because deranged, misinterprets the perfectly accurate impressions which reach it. As for the more important counterexamples to the theory, those of optical illusions, the paradigmatic case is the square tower which looks round at a distance. The images start out square, but because of the distance which they must travel through obstructive air they are rounded on arrival at the eyes (if this is too crude to be credible, substitute the image of a coin, oval due to perspective). Therefore vision is accurately registering the images as they are on arrival. This is a correct, not a misleading, view of a distant tower, and to call it a case of mis-seeing, says Epicurus (Sextus Empiricus, Against the Professors VII 206–10), is like saying that you are mishearing someone across the room just because you are not hearing their voice as it would sound inside their mouth.

The veracity of the senses is not, as many hold, impugned by their ability to conflict with each other. Strictly speaking, no two sensations are commensurable. If sight and touch differ about the shape of an object, that is because sight reports the shape of a colour patch, touch the shape of a body. If two visual sensations of the same object conflict, as in the case of the tower, that is because they are reporting two different kinds of colour patch, mediated by different quantities of intervening air.

The photographic reliability of the senses is what makes them not only true but a criterion of truth. However, when Epicurus appeals to the evidence of the senses as his ultimate criterion, it is not usually to these raw sensations, still awaiting interpretation by the mind, but to facts directly attested by them, for example, that things move, or that round objects move more readily than jagged ones. He regularly talks as if these interpreted sensations acquire the self-evident veracity which initially belongs only to the raw impressions.

The second criterion of truth is prolēpsis, or ‘preconception’ – a key term in ancient epistemology, first introduced by Epicurus. The prolēpsis of a thing is an instinctively acquired generic grasp of its nature, which enables you to recognize instances of it and is available for analysis in conceptual inquiries. Your prolēpsis of a human being, acquired unreflectively by accumulated past experience, enables you both to recognize humans when you meet them, and in the course of an inquiry to establish their essential features, for example, rationality and mortality. (When prolēpseis are said to be built up from past sensory experiences, these must again be interpreted sense–impressions, not raw ones.) Other prolēpseis appealed to in Epicurean arguments include those of god, body, utility and responsibility. Prolēpseis are taken to be common to all human beings, and therefore to function like a set of shared intuitions which we can hope to rediscover beneath our acquired false beliefs and to use as common ground for joint philosophical inquiry

The third criterion of truth is feelings (pathē), which generically divide up into just two kinds: pleasure and pain. As such, feelings certainly function as criteria of value, since Epicurus equates good with pleasant, bad with painful. But they also provide the introspective data on which Epicurus founds his psychological theory (Letter to Herodotus 63).

Citing this article:
Sedley, David. The criteria of truth. Epicureanism, 1998, doi:10.4324/9780415249126-A049-1. Routledge Encyclopedia of Philosophy, Taylor and Francis,
Copyright © 1998-2022 Routledge.

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