I believe science is performed by people and therefore subjective ie I haven't performed those experiments so I have to look at the experiments objectively.
You can't. No single human being can look at
anything objectively. The peer-review process is designed to reduce that human subjectivity to absolute minimal levels, and there's still always the understanding that, no matter how often a single theory is supported by independent studies and experiments, there is always a chance that in the future, new studies and experiments will end up demonstrating that the theory was wrong all along.
That is to say, you could read a scientific report on an experiment, attempt to replicate the experiment yourself, and submit your own report: that's the peer-review process at work. Anyone can do it.
When it comes to a spirit I am alraedy subjective so I am not sure it would be easy for me to be objective but I will try. If I theorize that a spirit exists then that explains why there is a voice in my head that is not my own yet is a part of me. It also makes it more rational to believe that the spirit can separate from the body and return to it as in the Resurrection of Jesus and it makes sense that a spirit can return in a new life as in re-incarnation.
This is problematic for a number of reasons. First of all, you're not theorizing anything; your making a hypothesis. That's the second point of the scientific method (the first point is asking a question). You're not adequately exploring any other possible sources for this voice in your head, in order to see whether that hypothesis is the correct reason, or if there's something else going on. Human memory is capable of recalling audio, and so these "voices" could be part of that: memories of conceptions being worded creatively by your subconscious self, in a voice that you heard before but can't remember the source of.