Today, 03:53 PM
The concept underlying the expression "1 + 1 = 2" is necessarily true for all people in all places at all times.
The concept underlying the expression "Joe is a murderer" is not necessarily true for all people in all places at all times.
All reasonable people, for all times and all places, must necessarily accept, accede to, and agree with the meaning expressed by the statement "1 + 1 = 2", regardless of whatever other particular set of symbols and system of nomenclature might be substituted at the "surface level" to denote the abstractions underlying the elements of the statement. IOW: It is always and everywhere unreasonable to reject the "ship" (i.e., the underlying meaning) of the expression "1 + 1 = 2", regardless of how the semantic deck chairs might be arranged.
On the other hand, when it comes to normative and/or transcendental matters (such as those relating to the concepts of "law" and "crime"), reasonable people can and will disagree - on an essential level much more fundamental than the transient contingencies of mere symbols and nomenclature - about what "ought" to be, or about how such things "ought" to be understood. With regard to such matters, one can certainly adopt presuppositions and definitions such that - from within the context of those "axiomatic" elements - certain propositions must necessarily hold true. But other reasonable people who do not share those presuppositions and definitions will not necessarily be compelled to accept, accede to, or agree with those propositions (or the "axiomatic" elements from which they are derived). IOW: One cannot reasonably reject the concept presently denoted by the expression "1 + 1 = 2", but one might reasonably reject the concept presently denoted by the expression "Joe is a murderer".
To put things in terms of Feynman's empirical/positive/non-normative discussion of magnetism, one cannot reasonably dispute that the opposite poles of magnets attract (regardless of what symbols, labels, and nomenclature one might arbitrarily adopt in order to describe or express the fact). The question of whether the opposite poles of magnets attract (and under what circumstances they might not) is an "objective" one, and is existentially bound up with the essential identity of "magnets" as distinct from other things - but the question of why they attract (in the sense in which Feynman means "why" in this particular instance) is not. To address the former question is to engage in "science" (specifically, "physics"). To address the latter question is to engage in "philosophy" (specifically, "metaphysics").
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