The first three of Arthur Schopenhauer’s Four Laws of Thought are pretty much the same as the classical three laws of thought. Schopenhauer added a fourth law that was basically for his Principle of Sufficient Reason.
- Excluded middle
- Sufficient reason
These Four Laws are often given in two flavors: the first, in fairly concrete terms of subjects and predicates, and the second, more glib in terms of existence and being and such (isness).
- A subject is equal to the sum of its predicates. Everything that is, exists. (Identity)
- No predicate can be simultaneously attributed and denied to a subject. Nothing can simultaneously be and not be. (Non-contradiction)
- Of every two contradictorily opposite predicates one must belong to every subject. Each and every thing either is or is not. (Excluded middle)
- Truth is the reference of a judgment to something outside it as its sufficient reason or ground. Of everything that is, it can be found why it is. (Sufficient reason)
The phrase ‘it can be found’ sounds like a constructive method rather than a mere existence proof, but the common theological technique that combines both by saying “everything happens for a reason” avers the reason to an ineffable deity. (I bet Schopenhauer would have disliked this view because from what I understand he was an atheist.)
Moving on, I would like to represent these four laws in even more concrete terms of logical expressions. In the following attempt, let a, b be subjects (or objects), and P, Q be predicates (or qualities):
- ∀a (a ≡ ∀P P(a))
- ∀a ¬∃P (P(a) ∧ ¬P(a))
- ∀a ∀P (P(a) ∨ ¬P(a))
- ∀a ∃b (b → a)
When detailed in this way, these four laws don’t seem very complete, or don’t quite form a unity, as implication and equivalence are each in only one of them. Even though it doesn’t help that criticism, perhaps one can succinctly say:
- Things can be reduced to (all) their qualities.
- Qualities are disjoint from their opposites.
- Qualities and their opposites are sufficient.
- Things are entailed by some thing (possibly same).
In addition, I quite liked this Goodread review which aligns Aristotle’s Four Causes with Schopenhauer’s Fourfold Root. So then:
- From Parts : Material Cause : Becoming : Identity
- For Functions : Final Cause : Knowing : Non-contradiction
- Into Structures : Formal Cause : Being : Excluded-middle
- By Actions : Efficient Cause : Acting : Sufficient reason
At some point, I need to understand the difference between the law of the excluded middle and the principle of bivalence.