in order to know that people generally speak the truth....we need...

 Thinking that testimony is generally reliable will either be viciously circular or else involve an insurmountable regress. 

For instance, in order to know that people generally speak the truth, I might need to rely on Bill’s testimony to confirm that what Alice said was true. 

But in order to know that Bill can be trusted, I might need to rely on Carly to confirm that he usually says true things. 

But to ensure that Carly typically speaks the truth, I will either need to rely on Alice or Bill to confirm this for me (hence the vicious circle), or else I will need to rely on on a fourth person like Donald (and hence the regress will continue). 

Thus, because there is no good way to acquire the non-testimonially based reasons in question, Global Reductionism problematically entails that we are rarely (if ever) justified in accepting what people tell us