On pages 45-6 of ITOE Ayn Rand wrote:
'Observe that all of the above versions of a definition of man were
true, i.e., were correct identifications of the facts of reality - and
that they were valid qua definitions, i.e., were correct selections of
distinguishing characteristics in a given context of knowledge. None
of them was contradicted by subsequent knowledge: they were included
implicitly, as non-defining characteristics, in a more precise
definition of man. It is still true that man is a rational animal who
speaks, does things no other living beings can do, walks on two legs,
has no fur, moves and makes sounds.'
However, on page 46 she also wrote:
'In this issue, an ignorant adult is in the same position as a child
or adolescent. He has to act within the scope of such knowledge as he
possesses and of his correspondingly primitive conceptual definitions.
When he moves into a wider field of action and thought, when new
evidence confronts him, he has to expand his definitions according to
the evidence, if they are to be objectively valid.'
The issue here lies with the phrase "expand his definitions." In fact,
if a definition were expanded, it would simply become larger and
encompass more and more essentials. But in this case, previous
essentials were discarded in favor of new ones. So in fact the
definition was not expanded at all, it was essentially altered to
encompass new knowledge about "man."