_De Fato_ 29.

Sec.Sec.40--42. Summary. The Academics have a regular method. They first give
a general definition of sensation, and then lay down the different
classes of sensations. Then they put forward their two strong
arguments, (1) _things_ which produce _sensations_ such as might have
been produced in the same form by other _things_, cannot be partly
capable of being perceived, partly not capable, (2) _sensations_ must
be assumed to be of the same form if our faculties do not enable us to
distinguish between them. Then they proceed. Sensations are partly
true, partly false, the false cannot of course be real _perceptions_,
while the true are always of a form which the false _may_ assume. Now
sensations which are indistinguishable from false cannot be partly
perceptions, partly not. There is therefore no sensation which is also
a perception (40). Two admissions, they say, are universally made, (1)
false sensations cannot be perceptions, (2) sensations which are
indistinguishable from false, cannot be partly perceptions, partly not.
The following two assertions they strive to prove, (1) sensations are
partly true, partly false, (2) every sensation which proceeds from a
reality, has a form which it might have if it proceeded from an
unreality (41). To prove these propositions, they divide perceptions
into those which are sensations, and those which are deduced from
sensations; after which they show that credit cannot be given to either
class (42). [The word "perception" is used to mean "a certainly known
sensation."]

Sec.40. _Quasi fundamenta_: a trans. probably of [Greek: themelios] or the
like; cf. [Greek: hosper themelios] in Sext. _A.M._ V. 50. _Artem_: method,
like [Greek: techne], cf. _M.D.F._ III. 4, Mayor on Iuv. VII. 177. _Vim_:
the general character which attaches to all [Greek: phantasiai]; _genera_
the different classes of [Greek: phantasiai]. _Totidem verbis_: of course
with a view to showing that nothing really corresponded to the definition.
Carneades largely used the _reductio ad absurdum_ method. _Contineant ...
quaestionem_: cf. 22 and _T.D._ IV. 65 _una res videtur causam continere_.
_Quae ita_: it is essential throughout this passage to distinguish clearly
the _sensation_ (_visum_) from the _thing_ which causes it. Here the
_things_ are meant; two _things_ are supposed to cause two _sensations_ so
similar