Deductive Logic

Deductive logic is reasoning from a known principle to an unknown.  This approach is illustrated by Archimedes who, speaking of the principle of leverage, stated if he were given a place to stand he would move the world.  In philosophy and religion, if we can determine a known principle (a place to stand), our task of determining truth is greatly simplified.  However, how do we know that a particular known principle is really true?  If we start questioning what we believe is a known principle, we eventually will encounter a fact that we cannot prove.  Consider the classical syllogism:

All men are mortal;

Socrates is a man;

Therefore, Socrates is mortal.

The known principle is “all men are mortal” but how do we know this is true?  As we saw in the last blog, we cannot use inductive logic to prove this statement.  While everyone we have observed on earth up to this point in time is mortal, we cannot prove that in the future we will not find someone who is immortal.  We must agree with Trueblood who states that deductive logic has no perfect right to its premise. [1]

Self Evident Truths

While inductive logic cannot prove any starting or known principle, there are some who claim the existence of self-evident truths or undeniable first principles which can be used to ground our logic.  Self-evident truths, by definition, cannot be proven.  An example of a self-evident truth is:  knowledge is possible.  If we deny it, we are affirming it because we evidently believe knowledge is not possible and that is knowledge.  Another example is:  There is error.  If we deny it, we are saying the proposition is in error which means there is error. [2]   W. P. Montague proposes a test for self-evident truths:  The truth of a particular principle is proved necessary if the denial of that principle would involve self-contradiction. [3]  The above two examples conform to his test.  While at first glance this sounds like a solution to our problem, it does not because as Trueblood notes, “it is not easy to find propositions which meet this rigorous standard”. [4]  With the few self-evident principles we have, it is difficult to construct a philosophy on them because we would need to employ human experience and logic to make any use of these principles.  As we have seen, human experience and logic have their flaws.


[1]   David Elton Trueblood, General Philosophy (New York:  Harper & Row:  1963), p. 107.

[2]   Trueblood, p. 88.

[3]   As quoted in David Elton Trueblood, General Philosophy (New York:  Harper & Row:  1963), p. 89.

[4]   Trueblood, p. 89.

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