Sam Harris says science can be used for moral reasoning. What be really means is that we can simply be consequentialists, and use Science as a tool to determine consequences. We can apply logic and reason to decisions but that’s not the same as using science. Science also happens to use logic and reason but it doesn’t make it the same thing. Astrology too uses logic and reasoning and it’s not a science any more than philosophy ever can be.
Sam Harris would say that what makes for a good life is good consequences. Yes science and neuro-biology can be used as tools to see what those varying consequences may be, but no consequentialist would deny that. Science is a tool but not an end in itself. For example science can never determine meaning or relative value , which are judgement calls that constitute a good life. It’s just one tool of many. All that he can claim is that te science are an effective tool for discovering different consequences in the consequentialist ethic. Science alone can’t even lead one to conclude that consequences should matter. That’s a value judgment. That suffering or well being ultimately matters is a value judgment.
We exist in an absolutely amoral universe. There’s nothing aside from reason and value judgements that we can turn to. Science alone cannot substitute for them. Science is just a tool which also happens to properly apply reason under normal circumstances. One can’t just say science when they mean reason. What is science? It’s a process of interpreting stimuli and using observation and experimentation to draw conclusions about material reality. This material reality does not include elusive immaterial entities like value and meaning so therefore by definition science cannot be used in this regard. If you claim that meaning of life is to optimize pleasure and value should be judged by optimization of pleasure alone, then you can say that science is the ultimate tool, but there’s nothing scientifically verifiable about these assumptions about morality in the first place.