Query:

I have been wondering about self-evident first principles.  They can’t be proven, because you can’t deduce them from other truths.  So what makes them true and self-evident?

Reply:

Good question.  Although self-evident first principles aren’t demonstrable, they don’t have to be, because they aren’t deniable either.  Willy nilly, we make use of them even in the attempt to deny them, so we haven’t succeeded in denying them after all.

For suppose someone suggested that the proposition “A statement can be both true and false in the same sense at the same time” is true (as some people do!)  By the very act of claiming that it is true, he would also be claiming that it is not false.  So he would still be relying on the fact that it couldn’t be both at once.  And it would be equally silly to argue that it is good to pursue evil, and evil to pursue good, because he would still be assuming that good is to be done and evil isn’t.

Another problem is what logicians call “explosion”:   From any contradiction whatsoever, every conclusion follows.  So if, for example, you say that the proposition “abortion is wrong” is both true and false – I’ve seen that one claimed – then it would follow there are fairies.  And that water flows uphill.  And that it doesn’t flow uphill -- whatever you want!  Logical reasoning would be kaput.

What makes first principles evident in themselves is that their predicates are “contained” or implicit in their subjects -- they merely draw out what their subjects mean already.  That’s how it works with all self-evident propositions.  For example, since man is a rational animal, of course he has a mind and a body.  Now the good is that which we naturally seek, so to know that something is good just is to know that it is to be sought and its opposite avoided.  And the truth is how things really are, so to know that something is true just is to know that it corresponds to what is and excludes what is not.

Notice then that each first principle has two forms:

I. The ontological form of the first principle of theoretical reason is that nothing can both be and not be in the same sense at the same time.

II. Its propositional form is that nothing can be both affirmed and denied in the same sense at the same time.

And

1. The ontological form of the first principle of practical reason is that good is that which all things naturally seek.

2. Its preceptive form is that good is to be done and pursued, and its contrary avoided.  This is also the first precept of natural law.

Just as statement (II) expresses (I) in a form adapted to demonstration, so statement (2) expresses (1) in a form adapted to deliberation.

I’m sure you’ll agree that from a certain point of view, all this becomes obvious.  Our minds are, so to speak, magnetized toward being and good.  These are the compass points that draw all thinking about what is and what ought to be done.  But if we don’t restate the obvious, we can get into a lot of trouble, so I’m glad you asked.