Rene Descartes was essentially saying we cant know anything for sure therefore we cant have real knowledge. Where Russel was more along the lines of its true we cant disprove certain things, but that doesnt make them likely. its entirely possible that we are just brains in a vat but its also highly unlikely and there is no reason to believe that is true. the famous tea pot thought experiment represents this well. what if i told you that between the earth and mars there is a tea pot revolving around the sun? the tea pot is so small that none of our telescopes can discover it? the claim isnt disprovable but it is highly