a square tower when seen from a distance may be judged round. The "replicas which leave the tower have
sharp edges, but 'as they travel through a large space of air, the air by frequent collisions blunts them' and
they become rounded. Even so, the tower does not look round: 'the stone structures look as though they
had been turned on a lathe - yet they do not look like stones which are nearby and genuinely round, but
seem to be, as it were, sketchy likenesses of them'. Again, 'Timagoras the Epicurean denied that when he
pressed his eye he ever seemed to see two little flames in the lamp - the falsity belongs to opinion, not to
the eyes.' Careful attention to the exact content of our impressions will show that illusions are due to the
mind's misreading, not to the eye's misleading.
As for conflicting perceptions, each is true - but true of only a part of the object perceived. 'Since
everything is mixed and compounded, and different things naturally fit different people ... people
encounter only those parts with which their senses are commensurate.' The water feels cold to me, warm
to you. We are both right, or partly right; for the water contains cold elements and warm elements, the
former affecting me and the latter you. Perception does not err.
The Epicurean defence of impressions was not mere quixotry. Epicurus asserted that 'if you reject
absolutely any perception ... you will confound your remaining perceptions too with that empty belief, so
that you will reject every criterion'. If any perception fails, all fail - and knowledge is lost. The Academics
were not satisfied, but they were more interested in the Stoics than in the Epicureans.
The focus of the dispute was the final clause in the Stoic definition of apprehensive impression: the
impression must be 'of a sort which could not derive from any other existing object'.
There are four points which imply that there is nothing that can be known, grasped or apprehended ...
First, some impressions are false; secondly, they cannot be apprehended; thirdly, if there is no difference
between two impressions it cannot be that one of them can be apprehended but the other not; fourthly, for
any true impression derived from the senses there is another adjacent impression which does not differ
from it at all and which cannot be apprehended.
If Chrysippus claims to possess a true impression, the Academics will offer him another impression which
is false but which Chrysippus cannot distinguish from the impression he claims to be true. Hence his
impression is not apprehensive; for it could have derived from something else, namely from the object
from which the indistinguishable false impression derives. The Academics produced pairs of eggs,
identical twins, real and wax apples, in an attempt to substantiate their claim that for every true
impression there was an adjacent false impression indistinguishable from it. The Stoics made defensive
manoeuvres. They claimed distinctions where the Academy saw none, they added an extra clause to their
definition of apprehensive impression.
Moreover, they took the battle into the Academic camp. Without belief, life is impossible; for we lose all
reason for action. The Epicureans made the same point. According to Lucretius, 'life itself is immediately
ruined if you are not prepared to believe your senses - and to avoid precipices'.
Science