"Science has got things wrong in the past though."

"How can we know anything is true?"

 

This is an attempt to deflect the course of the conversation by introducing an irrelevant point. This if usually to try and cover up a weakness in the argument at hand, so as to avoid having to admit "defeat". Rather than address this new topic in detail, I recommend asking them to keep that for a separate discussion and to focus on the issue at hand. Otherwise you can get taken down an endless garden path.