Special pleading is a form of spurious argumentation where a position in a dispute introduces favorable details or excludes unfavorable details by alleging a need to apply special considerations or exemptions from typical analysis.
These considerations or exemptions are pushed into the argument without proper critique or analysis. Essentially, this involves someone attempting to cite something as an exemption to a generally accepted rule, principle, etc. without justifying the exemption.
This fallacious argument is one of the most used in alternative medicine and other pseudoscientific areas, since, lacking any scientific evidence, they attempt to excuse the lack of evidence not because of implausibility, but because we lack the abilities to understand their special mechanism. Often, you will hear the phrase, “mainstream science just hasn’t uncovered the mechanism yet.”
Homeopathy cannot be tested with mainstream science because homeopathy requires a different kind of science to test its effects and to understand its unique mechanisms. When science catches up with homeopathy, everyone will see how it works.