TIP of the day (logic):To prove a negative you must ...

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MJ. Smith | Forum Activity | Posted: Wed, Mar 2 2016 11:36 PM

While Greta Christina may have made a major error by arguing "This is probably the best argument I have against God's existence: There's no evidence for it.", the fact that the argument is fallacious only proves that the argument is fallacious not that the conclusion is wrong. Another argument might be successful.

J.P. Moreland and W.L. Craig, Philosophical Foundations for a Christian Worldview:

If someone were to assert that there is an elephant on the quad, then the failure to observe an elephant there would be good reason to think that there is no elephant there. But if someone were to assert that there is a flea on the quad, then one's failure to observe it there would not constitute good evidence that there is no flea on the quad. The salient difference between these two cases is that in the one, but not the other, we should expect to see some evidence of the entity if in fact it existed. Moreover, the justification conferred in such cases will be proportional to the ratio between the amount of evidence that we do have and the amount that we should expect to have if the entity existed. If the ratio is small, then little justification is conferred on the belief that the entity does not exist. [For example] in the absence of evidence rendering the existence of some entity probable, we are justified in believing that it does not exist, provided that (1) it is not something that might leave no traces and (2) we have comprehensively surveyed the area where the evidence would be found if the entity existed..

(No, the book is not in Logos but it should be.)

What this means is that one form of argument that may be valid is in the form of:

  • If God (defined as N) exists, then the consequence Y should be observed.
  • Y is not observed
  • Therefore God (defined as N) does not exist.

This is a standard modus tollens argument which Wikipedia illustrates as follows:

Wikipedia: Modus tollens:

Consider an example:

If the watch-dog detects an intruder, the watch-dog will bark.
The watch-dog did not bark.
Therefore, no intruder was detected by the watch-dog.

Supposing that the premises are both true (the dog will bark if it detects an intruder, and does indeed not bark), it follows that no intruder has been detected. This is a valid argument since it is not possible for the conclusion to be false if the premises are true. (It is conceivable that there may have been an intruder that the dog did not detect, but that does not invalidate the argument; the first premise is "if the watch-dog detects an intruder." The thing of importance is that the dog detects or doesn't detect an intruder, not if there is one.)

In an argument of this form, the atheist/agnostic is forced to do two things:

  • define what they mean by "God" so we know what they are stating does not exist
  • provide observable consequences that follow if such a God does exist.

I find that many popular atheists define God in a way that I agree does not exist. Finding a common definition of God may then be the first step in a useful dialogue (or debate).

Some definitions of God, e.g. Tillich's "ground of being" easily lend themselves to observable proof - there are many existent observable things. But these nearly axiomatic definitions are rarely what the atheist is thinking of when they say "God does not exist".

Dawkins uses essentially a single attribute to define God:

Richard Dawkins in The God Delusion:

I am not attacking the particular qualities of Yahweh, or Jesus, or Allah, or any other specific god such as Baal, Zeus or Wotan. Instead I shall define the God Hypothesis more defensibly: there exists a supernatural intelligence who deliberately designed and created the universe and everything in it, including us.

The problem with this definition, is that Dawkins must either define the "observable consequence" as creation or admit his definition offers no such "observable consequence".

What has happened is that the most common means of proving a negative assertion, requires specific observations to be verifiably not present - you don't have diabetes because your glucose readings are consistently below the threshold for diabetes; there isn't "an elephant in the quad" ...

Orthodox Bishop Hilarion Alfeyev: "To be a theologian means to have experience of a personal encounter with God through prayer and worship."

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Sascha John | Forum Activity | Replied: Thu, Mar 3 2016 1:36 AM

My Problem mostly is, that the First Sentence is false and so the Rest also?

The Watch Dog

1. He is suppose to bark, but if he is dead, sleeping, run away or get feed by the intruder he will not bark.

3. The Dog could have detected the intruder but decidet not to bark for any Reason.

So, the real Question is Why should God prove to you his Existence if not even your Dog dit what you whantWink


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MJ. Smith | Forum Activity | Replied: Thu, Mar 3 2016 2:23 AM

Sascha, you are over-thinking it but in a good way.

The argument only says that IF the first two statements are true, the conclusion is true.

So if the dog is dead, sleeping, has run away etc. then he does not detect the intruder. (as in Wikipedia comments).

if the dog does not bark because the intruder fed him, then it's not true that the dog barks when he detects an intruder. He only barks some of the time so Statement one is false.

if the dog decides not to bark, again he only barks some of the time so Statement one is false.

A modus tollens argument says nothing about what happens if the premise(s) is false.


Why should God prove his existence?  <answer deleted as it is theological not logical>

Orthodox Bishop Hilarion Alfeyev: "To be a theologian means to have experience of a personal encounter with God through prayer and worship."

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