Aristotle, who lived from 384 BC to 322 BC, taught that
the heart contained the intelligence. Except for a few
guesses like this, he observed carefully. He described
the habits and the bodily structure (anatomy) of many
animals, birds and fish. Another Greek, Galen, who
lived in the second century AD, performed biological
experiments. Both careful observation of living things
and experiments are used today in the study of
biology. Galen observed and described the internal
anatomy of the human body.-He described many parts
that he saw in the body. But he was not always careful.
He sometimes described what he expected to see and
did not look carefully to see what was really there. For
centuries after Galen, people studied anatomy from his
books instead of trusting their own observations.