> out of curiosity, why would the default presumption be that other vertebrates feel pain, have thoughts, are conscious, while the default presumption in most o

> out of curiosity, why would the default presumption be that other vertebrates feel pain, have thoughts, are conscious, while the default presumption in most other sciences is that something is not true unless evidence is provided positively. Your question is foiled by semantics. The truth is the truth, regardless of phrasing of the hypothesis. For example, if my hypothesis is that fish do feel pain, I pre-establish experiment criteria, which if witnessed during testing would support the conclusion that the hypothesis is true; fish do feel pain. Similarly, if I start with the hypothesis that fish do not feel pain, establish my criteria and witness the empirical results passing said criteria, then the results support that the hypothesis is true (fish do not feel pain). So, it doesn't matter how the hypothesis is phrased, what matters is the test criteria and the empirical results against that criteria. You can arrive at the same conclusion either way. If you want to contest something, contest the criteria the researcher uses to base their conclusion. However, it gets more difficult to refute results when multiple experiments using a variety of different techniques & criteria all tend to point to the same truth.