If the person was capable of learning (it's not always so, particularly for older people), I'd start with explaining specific errors in reasoning actually exhibited by such confused replies, starting with introducing the rationalist taboo technique (generalized to assuring availability of an explanation of any detail of anything that is being discussed).

Here, we have overuse of "rational", some possibly correct statements that don't seem related ("Our not wanting to die is a bit of irrational behavior selected for by evolution. The universe doesn’t care if you’re there or not."), statements of outright unclear motivation/meaning/relevance ("The contrasting idea that you are the universe is mystical, not rational."). Then, some feign-sophisticated rejection of statements of simple fact ("The idea that you are alive “now” but will be dead “later” is irrational.").

(This won't work well in writing, you need to be able to interrupt a lot.)