[Serious] 'Counterargument - immediate fresh argument'---a forum game.

[arewethereyet]

WOW!  This is fun.

[Porcupine]

That one is true (all humans are mortal) but invalid. In order for it to be valid, you would need to reverse the clauses in Premise 1, because not everything that is mortal is human.

Yep it's an invalid inference, and formal fallacy, also known as Affirming the consequent.

Feel free to offer another argument as well.

Here's another from me:

Premise 1: If one knows of no evidence of God's existence then it is not rational for one to believe in God.
Premise 2: One knows of no evidence of God's existence.
Conclusion: Therefore, it is not rational for one to believe in God.

WOW!  This is fun.

I suspect that you made your comment sarcastically---and therefore non-seriously...---but I kudosed it because I actually seriously agree with it. For me, this really *is* fun.

[arewethereyet]

Seriously sarcastic.

[The Grand Nudger]

If the harbor gazette writes about it, a ship will sink off the coast.
The harbor gazette will write about it,
therefore a ship will sink off the coast.

[Porcupine]

If the harbor gazette writes about it, a ship will sink off the coast.
The harbor gazette will write about it,
therefore a ship will sink off the coast.

Invalid argument because 'if X happens then Y does happen", as a premise, isn't fulfilled with 'X will happen'.

Premise 1: If a great many fossils and DNA evidence is found then evolution is very probably true.
Premise 2: A great many fossils and DNA evidence is found---but God put them there to test our faith.
Conclusion: Therefore, evolution is very probably true.

[GrandizerII]

If the harbor gazette writes about it, a ship will sink off the coast.
The harbor gazette will write about it,
therefore a ship will sink off the coast.

Invalid argument because 'if X happens then Y does happen", as a premise, isn't fulfilled with 'X will happen'.

I see what you're saying, but I don't think the future tense indicated by "will" makes a difference here. The term "writes" seems to signify a future action in this case even if it's a present verb. If the first premise had "wrote" or "is writing", then I'd agree.

Premise 1: If a great many fossils and DNA evidence is found then evolution is very probably true.
Premise 2: A great many fossils and DNA evidence is found---but God put them there to test our faith.
Conclusion: Therefore, evolution is very probably true.

I say it's valid, despite the "but God" bit. Correct? It's a tricky one for me.

P1: If it is inevitable that I will do X, then I will do X.
P2: It is inevitable that I will do X
C: Therefore, it is a necessary truth that I will do X.

[Porcupine]

I say it's valid, despite the "but God" bit. Correct? It's a tricky one for me.

I intended it to be tricky This is part of the fun of this thread for me.

I think it just depends on how you interpret my premise and whether you think there are any enthymemes or not.

P1: If it is inevitable that I will do X, then I will do X.
P2: It is inevitable that I will do X
C: Therefore, it is a necessary truth that I will do X.

It depends how we interpret 'necessary' here. If the 'necessary truth' part means that it isn't contingent on existent hidden premises---enthymemes---that contradict the first two premises then the conclusion doesn't necessarily follow and, hence, the argument is invalid. But if the 'necessary truth' requirement doesn't contradict any of those premises, or if there are no such enthymemes, then the argument is valid (but not necessarily sound).