Sentence:  This urge to ensure the continued viability of UK science is balanced by warnings that such a system brings closer the day when computers will take over the running of the world and discard humanity. One such <WSD>gainsayer</WSD> is Bill Joy, inventor of the web language Java and chief scientist at computer giant Sun Microsystems. 
Meaning of the word marked with <WSD> tags:  A person who gainsays others; a disagreeable person.