Full Idea
Where Aristotle had 19 different inference rules (his valid syllogisms), modern propositional logic carries out deductions using just one rule of inference: modus ponens.
Clarification
'Modus ponens' says if p implies q, and p is true, then q must be true
Gist of Idea
Modern propositional inference replaces Aristotle's 19 syllogisms with modus ponens
Source
Keith Devlin (Goodbye Descartes [1997], Ch. 4)
Book Reference
Devlin,Keith: 'Goodbye Descartes: the end of logic' [Wiley 1997], p.80
A Reaction
At first glance it sounds as if Aristotle's guidelines might be more useful than the modern one, since he tells you something definite and what implies what, where modus ponens just seems to define the word 'implies'.