I meant look up Evidence.

And replicability is not a useful concept without focus - if you show people the same wavelength of light, yes, they will have different experiences of it. That isn't replicating. If you show one person a wavelength, and then show the same person the same wavelength, that is replicating. And you may yet get different experiences based upon context, or how they are feeling at the time. What does that tell you about "the difference between the nature of the systems under consideration" - ? Jack all - if you have different systems, studying one and then the other is not replicating the first study, it's studying something else. So your question doesn't make sense.

However, it is quite possible to study thought in science. You just have to focus in, so that what your study -note, not your results necessarily- IS replicable. That way, IF you get the same results, you HAVE evidence for something - whatever thing you were studying.
For instance, look at functional MRI studies, reaction-time studies, and other "hard" science in psychology, neuropsychology, psycholinguistics, etc. We are learning a whole lot about thought by studying it. We wouldn't if we just tried to believe in it, or pray about it, or look in ancient novels about it.