The Epicurean distinction between sensation and opinion, discussed yesterday (‘Sensation and judgment’), was succinctly explained by the second-century AD philosopher Sextus Empiricus in Book I of his work Against the Logicians, section 210.
According to this explanation, sense-perception (αἴσθησις) is non-rational (ἄλογος). It grasps what is presented to it but cannot make decisions about one presentation and another. The presentations (φαντασίαι) are ‘true’ but we have to form opinions (δόξαι) about them, and our opinions can be true or false. We can make correct judgments, or we can make bad judgments when we unnecessarily add to or subtract from the evidence of the presentations and so falsify our sense-perception.
This theory helps to explain not only how we can know about things but how we can err in our knowledge and how it can happen that people can come to different opinions about the same thing. The issue was much debated among philosophers of various persuasions.
Sextus Empiricus, Against the Logicians, Books I-II (also counted as Books VII-VIII of his Against the Mathematicians): see Hermannus Mutschmann (ed.), Sexti Empirici opera, vol. II: Adversus Dogmaticos libros quinque (Adv. Mathem. VII–XI) continens (Bibliotheca Scriptorum Graecorum et Romanorum Teubneriana), Leipzig, Teubner, 1914, ΠΡΟΣ ΛΟΓΙΚΟΥΣ Αʹ, §210, at pp. 50-51; Richard Bett (trans.), Against the Logicians (Cambridge Texts in the History of Philosophy), Cambridge – New York, Cambridge University Press, 2005.
Thoughts for the Day, July 17: ‘If you simply reject any sensation as unreliable, without dividing an opinion into anticipated confirmation and evidence already present (as detected via sensation, feelings and immediate mental representation), by this false view you will disrupt the meaning of the rest of your sensations as well, with the result that you will be rejecting all basis for evaluation. If you validate all the things that await confirmation in your opinion-forming thoughts along with those that do not, you will inevitably be mistaken, as you will have done away with all room for questioning in every decision as to the correct or incorrect interpretation of reality’ (Principal Doctrines 24).