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11023
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The logical connectives are 'defined' by their introduction rules [Gentzen]
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Full Idea:
The introduction rules represent, as it were, the 'definitions' of the symbols concerned, and the elimination rules are no more, in the final analysis, than the consequences of these definitions.
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From:
Gerhard Gentzen (works [1938]), quoted by Stephen Read - Thinking About Logic Ch.8
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A reaction:
If an introduction-rule (or a truth table) were taken as fixed and beyond dispute, then it would have the status of a definition, since there would be nothing else to appeal to. So is there anything else to appeal to here?
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11213
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Each logical symbol has an 'introduction' rule to define it, and hence an 'elimination' rule [Gentzen]
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Full Idea:
To every logical symbol there belongs precisely one inference figure which 'introduces' the symbol ..and one which 'eliminates' it. The introductions represent the 'definitions' of the symbols concerned, and eliminations are consequences of these.
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From:
Gerhard Gentzen (works [1938], II.5.13), quoted by Ian Rumfitt - "Yes" and "No" III
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A reaction:
[1935 paper] This passage is famous, in laying down the basics of natural deduction systems of logic (ones using only rules, and avoiding axioms). Rumfitt questions whether Gentzen's account gives the sense of the connectives.
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19428
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Because something does exist, there must be a drive in possible things towards existence [Leibniz]
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Full Idea:
From the very fact that something exists rather than nothing, we recognise that there is in possible things, that is, in the very possibility or essence, a certain exigent need of existence, and, so to speak, some claim to existence.
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From:
Gottfried Leibniz (On the Ultimate Origination of Things [1697], p.347)
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A reaction:
I love the fact that Leibniz tried to explain why there is something rather than nothing. Bede Rundle and Dale Jacquette are similar heroes. As Leibniz tells us, contradictions have no claim to existence, but non-contradictions do.
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5047
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The world is physically necessary, as its contrary would imply imperfection or moral absurdity [Leibniz]
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Full Idea:
Although the world is not metaphysically necessary, such that its contrary would imply a contradiction or logical absurdity, it is necessary physically, that is, determined in such a way that its contrary would imply imperfection or moral absurdity.
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From:
Gottfried Leibniz (On the Ultimate Origination of Things [1697], p.139)
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A reaction:
How does Leibniz know things like this? The distinction between 'metaphysical' necessity and 'natural' (what he calls 'physical') necessity is a key idea. But natural necessity is controversial. See 'Essentialism'.
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7667
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There are two sides to men - the pleasantly social, and the violent and creative [Diderot, by Berlin]
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Full Idea:
Diderot is among the first to preach that there are two men: the artificial man, who belongs in society and seeks to please, and the violent, bold, criminal instinct of a man who wishes to break out (and, if controlled, is responsible for works of genius.
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From:
report of Denis Diderot (works [1769], Ch.3) by Isaiah Berlin - The Roots of Romanticism
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A reaction:
This has an obvious ancestor in Plato's picture (esp. in 'Phaedrus') of the two conflicting sides to the psuché, which seem to be reason and emotion. In Diderot, though, the suppressed man has virtues, which Plato would deny.
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19429
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The principle of determination in things obtains the greatest effect with the least effort [Leibniz]
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Full Idea:
There is always in things a principle of determination which is based on consideration of maximum and minimum, such that the greatest effect is obtained with the least, so to speak, expenditure.
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From:
Gottfried Leibniz (On the Ultimate Origination of Things [1697], p.347)
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A reaction:
This is obvious in human endeavours. Leibniz applied it to physics, producing a principle that shortest paths are always employed. It has a different formal name in modern physics, I think. He says if you make an unrestricted triangle, it is equilateral.
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