Good point. The question is, though: is there any way we humans can avoid having to rely on making definitions at some level?
I think the answer is probably "no." For example, if we conside Physics in conjunction with Godel's Incompleteness Theorem, then we are left with something similar to what Kant said, and we're left to rely on definitions.
(BTW, according to the link, Godel's theorem seems to be an example of a negative proof...)
Heisenberg's uncertainty principle would be another example of a "negative proof" - the better you know the position of an object, the less you know its momentum, until at the ultimate point you know all about one and nothing about the other.
Hank Kerchief has pointed out that the way get a believer to understand disbelief is through appropriate use of definitions. My reply was that if the definition is nebulous enough, ala Deists' ideas about God, the definitional approach might not work.
To prove a negative actually means someting different. It is possible to prove that something does not have this or that attribute, (e.g. a number system does not have completelness, for example) but not possible to prove something does not exist. Personally, I am not convinced this is true. Take jimt's example: If I postulate the existence of gnorixes, for example, who control the rate of corrosion on pennies, can you prove they don't exist?
They can, in fact, be pretty easily disproved, because the nature of the corrosion of pennies is well understood, and we know exactly how that rate is determined. Occam's razor precludes another redundant cause for the same thing, however wonderully imagined it is.
This was my point about definitions. Most of the things that one supposedly cannot disprove are only difficult to disprove because what they are supposed to be is left vague. As soon as a postulated existent is actually defined, it is usually fairly easy to determine if it assumed attributes are possible.
I am surprised by those who argue against God that this argument is never made. In every day life, the theists attribute everything from the country's economy to the health of their dog to the action of God. Most of these things, however, already have complete explanations, just as the corrosion of pennies do. Attributing an additional cause, like the action of God, to events the cause of which is already fully understood is a violation of Occam's Razor.
The cause of the house burning down cannot be both the act of the arsonist and the act of God, unless God is the arsonist, of course.
Hank