Biology was a natural pursuit for Aristotle, given his
family's medical background. Along with his achievements in logic,
his work in biology constitutes his greatest and longest-lasting
success. He identified approximately 495 different species of animals,
some of which were alluded to briefly and others that were studied
in considerable depth. A significant part of his achievement was
simply the sheer amount of data that he collected, but he is also
praised for the skill and care with which he organized the data,
along with the insights he offered.
For example, he recognized that the cetaceans possessed
mammalian characteristics–a fact that all other writers overlooked
until the sixteenth century. He showed great accuracy in his depiction
of the chicken embryo. Such achievements might mean little for
the lay reader, but later generations of biologists have expressed
great admiration for the level of depth and accuracy that he attained. There
is no doubt that Aristotle was far ahead of his time.
His central work in biological studies was titled The
History of Animals. Aristotle draws the most important
distinctions between animals with and without blood and between
viviparous (reproducing offspring within the female's body, as
generally the case with mammals) and oviparous (reproducing through
the hatching of eggs) animals. He paid considerable attention to
the questions of reproduction and heredity, determining what factors
contribute in what ways. Aristotle's teleology played a particularly
important role in his biological studies. He believed that no organ
was given to an animal without a purpose. Thus he was careful to
distinguish between final and variable characteristics. Final characteristics were
those essential to an animal species, while variable characteristics
consisted of qualities that develop rather than being naturally
endowed.
For Aristotle, biology and psychology were intertwined,
much more so than we would view them today, and he treated the
two subjects as one science. The purpose of psychology was to discover the
attributes and essence of the soul (translated from the Greek work psyche).
Aristotle struggled to come up with a single definition of the
soul and concluded that none existed. On the other hand, the variations
in the kinds of souls were not so different that some common ground
could not be ascertained. Aristotle therefore arranged a series
of various forms that become increasingly complex, so that each
form of the soul possesses the qualities of all those that precede
it in order. The most basic soul is nutritive, which exists in
all living things, including both plants and animals. Beyond the nutritive
soul is the sensitive soul, possessed by all animals. This category
can itself be broken down into the same kind of hierarchy, in which
touch is the most basic sensation. A sensitive soul is capable not
only of perception but also of desire, since it can feel pleasure
and pain. Moreover, an animal can possess two additional faculties that
are not necessarily found in all: the first he calls imagination, which
also includes the faculty of memory and is an extension of the cognitive
aspect of the animal; the second is the faculty of movement, an
extension of the appetitive side. Human beings of course possess
the most complex soul, which exercises the faculty of reason. Aristotle
justifies this hierarchy by showing that the faculties are ordered
by their necessity, nutrition being the most fundamental and reason
contributing not so much to sustenance as it does to well-being.
One key issue that Aristotle raises is the relationship
between soul and body. He views them as inseparable and makes the
analogy that the soul is to the body as form is to matter. In other
words, the soul is the primary actuality of the body, providing
the body with its essential character and therefore is inseparable
from it.
Aristotle's account of such faculties as common sensibility
and imagination generally reveal the limitations of his knowledge
of physiology. In particular he is concerned with the faculty of
perception, which is linked most closely to sight but has a more
general application that plays a role in all judgments related
to the senses. Aristotle also wrote a series of supplemental treatises
on the difference between memory and recollection, the acts of
sleeping and waking, the act of dreaming, and more. But these are
read generally only for historical interest. While Aristotle may
have viewed biology and psychology as one science, the respective
lasting significance of each demonstrates the vast difference between
them. Aristotle's psychology is based largely on speculation that
has since been discarded based on improved understanding and technology,
while his contributions to biology were grounded in skilled observations
interpreted with keen insight that took centuries to surpass.
