Definition of concepts

Hello Rabbi, I heard the first lesson in the above series and two questions came to my mind about what was said. I would be happy to clarify:
A. The rabbi said that Aristotle defines by genus and species, when the rabbi explained that the first is the significant matter, and the second is a unique matter, distinguishing it from the other species in the genus. For example, you brought up that a person is a ‘living being that speaks.’ I was unable to understand why it is not enough to define by ‘speaking’, as if it were creating a group of ‘persons’ only. That is, there is no more ‘speaking’ in a genus that is not ‘living.’ Unless the purpose of the definition is not only to point out a particular group and cut it off from the rest, but also to characterize it, but I do not think the rabbi mentioned anything like that.
on. The rabbi concluded by saying that there is a difference between Aristotle and Plato, whether the definition is trying to recognize something that exists (the idea of ​​_) or a useful fictitious process. I didn’t quite understand what it meant that there is a real idea, only that it was not realized, that is, why isn’t it really simpler that a definition is the combination of a group of characteristics into one word.
Thank you very much.