There were four classic laws of thought recognised in European thought of the seventeenth and eighteenth century, which held sway also during nineteenth century (while subject to greater debate). They were:
In this formulation they are due to Leibniz. In the approach of Continental rationalism in general, these are supposedly clear and incontestable axioms.
The first two are from Aristotle and scholastic logic; the other pair are Leibnizian principles . As turned out to be the case with another such (the so-called law of continuity ), they stand for matters which, in contemporary terms, are subject to much debate and analysis (respectively on determinism and extensionality). In a sense that marks what happened in logic in the nineteenth century and particularly after Frege (who was much influenced by these formulations). Such laws were supposed to be of basic, pedagogic value, rather than challenges to the intellect. This attitude only dropped out some time early in the twentieth century, as can be seen by Bertrand Russell's allusion to them in a 1911 work (at which point there were three).
The laws of thought were particularly influential in German thought; in France the interpretation of the Port-Royal Logic tended to dispel their mystique. Hegel quarrelled with the law of indiscernibles while putting together his own 'logic ' — but as a current matter rather than an obsolete issue.
The title of George Boole's 1854 treatise on logic, An investigation on the Laws of Thought indicates a fresh start. These laws are now part of boolean logic; where the first two on the list come down to saying there are two and only two truth values. The second pair are ignored, at the algebraic level, absent second-order logic.
See also
Last updated: 05-17-2005 01:56:16
