But it is a received principle that error as well as knowledge in
respect to contraries is one and the same.
That perceiving and practical thinking are not identical is
therefore obvious; for the former is universal in the animal world,
the latter is found in only a small division of it. Further,
speculative thinking is also distinct from perceiving-I mean that in
which we find rightness and wrongness-rightness in prudence,
knowledge, true opinion, wrongness in their opposites; for
perception of the special objects of sense is always free from
error, and is found in all animals, while it is possible to think
falsely as well as truly, and thought is found only where there is
discourse of reason as well as sensibility. For imagination is
different from either perceiving or discursive thinking, though it
is not found without sensation, or judgement without it. That this
activity is not the same kind of thinking as judgement is obvious. For
imagining lies within our own power whenever we wish (e.g. we can call
up a picture, as in the practice of mnemonics by the use of mental
images), but in forming opinions we are not free: we cannot escape the
alternative of falsehood or truth. Further, when we think something to
be fearful or threatening, emotion is immediately produced, and so too
with what is encouraging; but when we merely imagine we remain as
unaffected as persons who are looking at a painting of some dreadful
or encouraging scene.
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