Carneades (c.214–129 BCE)

carneadesCarneades was perhaps the most prominent head of the
skeptical Academy in ancient Greece. Following the example of
Arcesilaus, who turned the Academy in a skeptical direction, Carneades
developed an array of arguments against the dogmatic positions upheld
by other philosophers, particularly the Stoics. He went beyond
Arcesilaus in several respects, however. Instead of simply arguing
against the positive positions of other philosophers, Carneades also
set forth arguments of his own in favor of views that sometimes had
never been defended before–not in order to establish their truth, but
simply to counterbalance the arguments of the dogmatists and show that
none of their conclusions can be conclusively established. In doing
so, Carneades made important contributions to several philosophical
debates. Carneades also set forth a more detailed skeptical criterion
of what to believe, to pithanon which means either the "plausible" or
the "probable."

1. Skeptical Practice

Carneades continued the skeptical academy's attack upon Stoic
epistemology. Arcesilaus had argued against Zeno, the founder of
Stoicism, that no sense-impressions could provide a firm foundation
for knowledge, since sense-impressions are always fallible. Carneades
maintained this criticism against refinements in the Stoics' theory
made by Chrysippus, the head of the Stoa at his time. But Carneades
went beyond criticizing the arguments of other philosophers by trying
to propound equally convincing arguments for incompatible conclusions,
which would have the effect of leaving his interlocutor suspending
judgement as to which is true. For instance, while on a mission to
Rome with the heads of two other philosophical schools, Carneades gave
an eloquent defense of traditional views on justice one day, and the
next day offered an equally eloquent attack on those same views.
(Unamused traditionalist Romans expelled the philosophers from the
city as a result.)
2. Contributions to Philosophical Debates

In arguing for contrary positions, Carneades sometimes came up with
novel positions or arguments. For instance, Carneades gave a taxonomy
of different possible candidates for what the highest good could be,
and in so doing, came up with possibilities not canvassed by previous
philosophers. He also defended original views in the debate between
the Stoics and Epicureans on human freedom, determinism, and the
truth-values of statements about the future. Against both Epicurus and
the Stoics, Carneades argued that no deterministic consequences follow
from the principle of bivalence (the principle that for any statement
P, either P is true or P is false). That is because, even if it has
always been true that e.g., I will brush my teeth tomorrow, that does
not imply that there are "immutable eternal causes" which will bring
it about that I will do so. It can be true now simply in virtue of the
fact that brushing my teeth is actually what I will freely choose to
do. Similarly, Carneades said that Epicureans can defend human freedom
from causal determinism without positing a random atomic swerve. A
person can be the cause of his actions by a "free movement of the
mind", without there being antecedent causes that necessitate that the
agent will do what he does. This is reminiscent of the theories of
"agent-causation" later propounded by writers like Chisholm.
3. Practical Criterion: To Pithanon

Carneades also developed a detailed skeptical criterion, to
pithanon–which can mean either "the plausible" or "the probable."
Sense-impressions can never be a sure guide to truth, thought
Carneades, but some are still more convincing to us than others–some
seem plausible, and others not. We need not stop there however–we can
make further investigation of convincing impressions to see if they
stand up or not, as well as seeing whether they cohere with our other
sense-impressions.

Exactly how to understand Carneades' criterion was controversial even
in his own day. Carneades left no writings, other than a few letters,
and Clitomachus, who was Carneades' closest associate and succeeded
him as head of the Academy, said he did not know what Carneades really
thought. Two questions are: (1) Are pithanon beliefs supposed to be
more likely to be true (as Cicero and Philo thought), or simply more
plausible to the person who accepts them? (2) Is Carneades advocating
to pithanon in his own voice as a criterion that a skeptic could use,
or is he simply employing it in service of his arguments against the
Stoics, without being committed to it himself?