def contrary
  if @quantity.label  == "particular"
    abort("There is no contrary for this type of propostion. Try subcontrary")
  end
  quality = @quality.opposite

  if @truthvalue
    truthvalue = !@truthvalue
  elsif !@truthvalue
    truthvalue = "unknown"
  end

  contrary = Proposition.new(@quantity, @subject, quality, @predicate, truthvalue)
  return contrary
end