  Boole did not have the modern view that a propositional formula can be considered as a function on the truth values \(\{\rT, \rF\}\), taking  values in \(\{\rT,\rF\}\).   The function viewpoint gives us an algorithm to determine which constituents are to be summed to give the desired elective expression, namely those constituents associated with the cases for which the propositional formula has the value \(\rT\).  