Human skin gives off an array of gaseous substances, including carbon dioxide and lactic acid, both of which attract mosquitoes. However, Support neither of these two substances, whether alone or combined with one another, will attract mosquitoes as much as a bare human arm will, Support even in complete darkness, where a mosquito has no visual cues. Therefore, Conclusion some other gaseous substance given off by human skin also attracts mosquitoes.
Human skin gives off a third substance that attracts mosquitoes, because when the two substances known to attract them is separated from skin, the human skin will still attract mosquitoes more than the two substances sans skin.
This is a causal argument, and we should immediately be suspicious of alternative cause. It doesn’t have to be a third gaseous substance, it could be something else that strongly attracts the mosquitoes to human skin. There are so many potential alternative causes, and for each one, knowing it is not eligible as alternative cause would be a necessary assumption.
The reasoning in the argument requires which one of the following assumptions?
Mosquitoes do not communicate with one another.
It doesn’t matter if mosquitoes communicate with each other or not. Whether it’s happening or not, we can’t assume anything about that communication.
Mosquitoes are not attracted to humans by body heat.
This is one of the alternative causes that we predicted. This must be true, because if it isn’t, then body heat is the alternative explanation for mosquito attraction rather than some gaseous substance.
Human skin gives off gaseous substances in greater amounts during the day than during the night.
It is not necessary that humans give off more gaseous substances at different times of the day, no matter how it’s distributed (day vs. night, etc.).
Mosquitoes are no more successful in finding a bare human arm in darkness than in light.
If negated, mosquitoes are more successful in finding an arm in darkness than in light. This doesn’t ruin the argument, because a third substance could still be the reason for this.
Also, darkness was mentioned in the argument to establish that the strong mosquito attraction is present for a human arm regardless of light.
Human skin never gives off any gaseous substances that repel mosquitoes.
This isn’t necessary—it’s perfectly fine if humans sometimes repel mosquitoes. That doesn’t harm the conclusion that one of the reasons we attract them is a third gaseous substance.
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