From ancient to new Stoicism: II—Stoic logic
A conceptual map of where Stoicism came from and where it may be going
We are in need of updating Stoicism for the 21st century and beyond. But in order to do that, we want a clear sense of what ancient Stoicism was about. In the first entry in this series we have looked at Stoic physics, one of the three areas of study in classical Stoic philosophy. This time we examine (briefly!) The second area: Stoic logic.
Don’t worry, this isn’t going to be a boring, heavily technical treatment. Then again, logic is fundamental not just for the Stoics, but for every thinking person. Here is how Epictetus explained why:
“Someone in the audience said, ‘Convince me of the usefulness of logic.’ ‘Shall I prove it to you?’ ‘Yes, please.’ ‘Then I’d better use a demonstrative argument, hadn’t I?’ His interlocutor agreed, and Epictetus went on, ‘So how will you know if my argument is fallacious?’ The man said nothing. ‘Do you see,’ Epictetus said, ‘that you are yourself admitting that logic is necessary, since without it you can’t even find out whether or not it’s necessary?’” (Discourses, 2.25)
Right? Okay, then, let us proceed, like last time, using as a guide the excellent article on Stoicism by Marion Durand and Simon Shogry, published in the Stanford Encyclopedia of Philosophy. You can go there if you need to dig deeper.
Rhetoric and dialectic
To begin with, Stoic logic included not just formal logic as we understand it today, but a broad range of disciplines related to sound thinking, including epistemology, grammar, philosophy of language, and rhetoric.
The two major branches of Stoic logic were rhetoric and dialectic. The first one was defined as “the science [or knowledge, epistêmê] of speaking well,” while the second one was “the science [or knowledge, epistêmê] of what signifies and what is signified” (Diogenes Laertius, Lives of the Eminent Philosophers, 7.42)
An important Stoic innovation was the introduction of what they called lekta, usually translated as “sayables.” The term covers everything that can be said, and therefore refers to the content of our thoughts, to our rational impressions, and to our judgments—pretty much what really matters to the Stoic.
The Stoics thought that the only bearers of truth in logic are propositions, often referred to as “assertibles,” a subset of sayables, since not everything that can be said is a bearer of truth. For example, is it true that unicorns have one horn? For the Stoics the question is undecided, since there are, in fact, no unicorns. The question itself, then, is a sayable but not an assertible.
Notice that a Stoic assertible can change truth value depending on the circumstances. If I say “it is day now” and it is in fact, day, the proposition is true. But it won’t be true tonight, and it will be true again tomorrow morning.
Stoic logic developed the truth conditions for combinations of assertibles: conjunctions (p and q), disjunctions (p or q), and conditionals (if p then q).
There is an important difference between Stoic logic and its more familiar Aristotelian counterpart: the Stoic system is one of propositional logic, while the Aristotelian one concerns predicate logic. That is, the Stoics were focused on the logic governing sentences, while Aristotle was occupied with the logic of terms. The two are not mutually exclusive and Stoic logic was much studied during the Middle Ages and Renaissance, representing the forefront of logical scholarship until the introduction in the 19th century of Gottlob Frege’s predicate logic. Technically, predicate (or first-order) logic is founded on propositional (or zeroth-order) logic. [1]
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