That is an intriguing statement. How do we then determine the "essence" of something. And what exactly is the "essence" of something?
Essence is a catch all for the physics of something. ie. physical composition, properties, morphology, interaction abilities and any other abilities. It's a nonspecific, all inclusive term, so that even what might not be known about the entity is referred to.
Re: Behavior does not determine the essence of something.
"How do we then determine the "essence" of something."
Behavior can be used to determine properties, or general abilities, but is does not determine what the underlying physical entity is. In the context the statement was made, the meaning of the statement is that even though behavior does change, it does not indicate, or determine the particular underlying physics. The underlying physics of the entity allow for the range of behaviors observed in the particular interaction events observed. A range in, or variation of, or differences in observations does not necessarily indicate a change in underlying physics.
The same can be said for rational beings. If an individual displays different behaviors, it does not mean the individual changes from one instance to the next. IOWs the particualr behavior doesn't determine who the individual is. The underlying values, or other references for the behavior, that are part of and are characteristic elements of the personality are what give rise to the behavior, and they are part of the essence of the person.