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What is the Objectivist explanation of how we know modus ponens?

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This question of William’s has been very fertile.

In his Intermediate Logic (1997), David Bostock argues there is a circumstance to be mentally entertained, a circumstance that has as its result that Modus Ponens would not be a valid rule of inference. Because of that result, he concludes that that circumstance is illogical (350–54). Because the circumstance pertains to empty referent for referring terms in logical relations, I incline to think all the more that knowing Modus Ponens is contained within and should be isolated within knowing consciousness is identification, where logic is understood to be a certain subdivision of verbal consciousness as identification. What then are the particulars of how we know the logic subdivision of such consciousness? Among those particulars should be how we know validity of Modus Ponens.

As observational givens that William mentions, I think for Rand’s epistemology as it leads to knowledge of logical principles, we must start with verbal reports of observation such as “this pen still has ink” and “this board is less bowed than that one.” How we know such observational reports of ours are true when they are true is one layer of epistemology. How we know logic is a further layer of epistemology, and how we know Modus Ponens has to be part of that further layer of hows.

Rand gestured in her epistemology that there are significant relations between (i) observation and elementary conceptual processes concerning observations and (ii) processes of induction and deduction (ITOE 28). That variety of induction would be most plausibly the sort of induction we know as abstractive induction (also known as intuitive induction). There was an attempt to expand on this gesture of Rand’s in the first chapter of David Harriman’s book The Logical Leap (2010), but it discussed the relation of observation, conceptualization, and ampliative induction, not abstractive induction. And the latter is what is relevant to how we know deductive logic.

Edited by Boydstun

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It won't answer the title question, but to somebody enough interested in modus ponens I recommend Bland Blanshard's The Nature of Thought, Vol 2, the chapter Formalism and Necessity.

The Feb 1963 The Objectivist Newsletter included a favorable book review of Blanshard's Reason and Analysis by N. Branden. That book also addresses modus ponens ("p implies q"), but not as much. 

 

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