"Doubt it" is pretty much a clear invitation to present arguments.
Present an argue against what? “Is not!” doesn’t give a lot to work with. “Doubt it” does a fine job of presenting one’s biases, but it is most certainly not an invitation to stimulating discussion, quite the opposite.
Present an argue against what? “Is not!” doesn’t give a lot to work with. “Doubt it” does a fine job of presenting one’s biases, but it is most certainly not an invitation to stimulating discussion, quite the opposite.