Rav Tzvi Hirsch ben Yaakov Ashkenazi, also known as the Chacham Tzvi, was born in 1660 and died in 1718(His son was Rav Yaakov Emden). This response states that a beating heart is a requirement for life. But clearly the Chacham Tzvi was not talking about a case of brain death where the person (or chicken as in this case) can’t breathe. Those who claim that brain-stem death is not halachic death, quote this responsa to support their position because it put emphasis on the heart. But those who accept BSD as halachic death point out that while the Chacham Tzvi writes one needs a beating heart to live, and it necessary for life, he does not write that a beating heart alone is sufficient in and of itself. When he writes the “soul is in the heart” it is rhetorical flourish to indicate that without a beating heart a person or animal can’t live. But again, that is not say it is sufficient for human life. A analogy would be that an airplane needs an engine to fly and to be called an airplane. But that does not mean if you have a working engine that you have an airplane.