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You now have to tools to appraise arguments to the level of detail offered by predicate logic.
Let us run through how these might be used with a long and difficult example.
Consider the argument
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If a derivation contains a line of the form
n (<variable>)<scope> <any justification>
then a line of the form
<<scope>[<term>/<variable>]> 'n UI'
may be added to the derivation, provided that <variable> is free for <term> in the <scope> ie that the substitution [<term>/<variable>] is legitimate .
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The material explained here is conceptually at a higher level than the earlier tutorials; it also covers relatively rare cases.
Herrick[2000] The Many Worlds of Logic Part IV
To understand the concepts of scope, free, bound, and free for. To learn of the restrictions on the quantificational rules
The semantics of relations proceeds in much the way one would expect-- the new item that has to be taken account of is the order of the terms (because, for example, Tab is not at all the same thing as Tba -- Arthur being taller than Beryl is not the same as Beryl being taller than Arthur).
Let us start with an Interpretation
Interpretation 1
Universe= {a,b}
F={a}
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Herrick[2000] The Many Worlds of Logic Chapter 16
[The core of what follows is from Leblanc and Wisdom [1972] p.117 and f.]
‘Everything is lovely’ we understand as meaning ‘Whatever thing you may choose, it is lovely’ and this becomes(x)Lx
Herrick[2000] The Many Worlds of Logic Chapter 17
Thus far we have considered only 'monadic' predicates-- our atomic formulas consist of a predicate followed by only one term-- for example, Fx. But in English we regularly encounter dyadic predicates or relations. For example, 'Arthur is taller than Bert' cannot be symbolized with the tools we have used so far; what is needed is a relation to represent '...is taller than ...' Txy, say, and then the sentence would be symbolized Tab.
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Herrick[2000] The Many Worlds of Logic Chapter 18
Existential Instantiation permits you to remove an existential quantifier from a formula which has an existential quantifier as its main connective.
The circumstance that Existential Instantiation gets invoked looks like this.
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Herrick[2000] The Many Worlds of Logic Chapter 18
There is a rule for adding a Existential Quantifier. This permits the step illustrated by the following proof fragments.
5 Fa 6 (∃x)Fx 5 EG
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Herrick[2000] The Many Worlds of Logic Chapter 18
There also is a rule for adding a Universal Quantifier. This permits the step illustrated by the following proof fragments.
5 Fx 6 (x)Fx 5 UG
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Herrick[2000] The Many Worlds of Logic Chapter 18
To understand the concepts of scope, free, bound. To meet substitution and a simplified version of the rule for removing a Universal Quantifier.