If (it is sunny) then (it is daytime). [a logical conclusion, and one you can always make]
Not (sunny today). [a fact, a negation of Q]
Therefore, not daytime? [you cannot conclude that]
BZZZ.. It's merely a cloudy day.
You cannot always disprove P by negating Q, unless P is all Q depends on.
You don't know what you're talking about.
What you posted there was not a modus tollens. It was, in fact, a fallacy which is called
"denying the antecedent".
What
I posted was this:
If P, then Q. (If it is sunny, then it is daytime)
Not Q. (It is not daytime)
Therefore, not P. (Therefore, it is not sunny)
That is a
modus tollens. What
you posted was this:
If P, then Q. (If it is sunny, then it is daytime)
Not P. (It is not sunny)
Therefore not Q (Therefore, it is not daytime)
That is a fallacy, as you say. But it's a fallacy that
you committed--not me.
Or you are blind. Or you are looking in the wrong direction. Or the elephant is inside a big box. etc.
This is a red herring.
The modus tollens is a
valid argument, whether you accept its premises, or not.
It may not be a
sound argument, if one or both of its premises are false. But anyone who knows anything about logic knows that. In fact, I said as much myself:
These are both valid arguments. If both of their premises are true, then their shared conclusion must follow. And since, in both cases, the second premise is obviously true, they can only be unsound if the first premise is false.
Emphasis added.
I'm assuming that you understand the distinction between validity and soundness. But since you don't seem to know the difference between denying the consequent and denying the antecedent, or even the difference between P and Q, I suggest you look it up.