Conscientious writers try to avoid making unfair assumptions that stereotype or over-generalize people or issues. At the same time, however, every argument conceals some implicit assumption that can't be helped. For example, the argument "An umbrella provides the best defense in wet weather" assumes that protecting one's self against rain is a desirable pursuit. Most readers will find this sort of assumption reasonable, though. To find the assumptions in a thesis, look for the ideas it takes for granted.
Locate the argument in the thesis. For example, in the claim "An umbrella provides the best defense in wet weather," the argument is that the umbrella is the best choice, in the opinion of the writer. You can double-check if the thesis is arguable by determining if it offers a contrary argument. In opposition to the above argument, one could claim that umbrellas do not provide the best defense; this confirms that it is indeed the argument.
Find the assumption inherent in the thesis. The assumption is the starting point of the thesis, or what it bases itself on. After taking out the argument, the assumption is what remains. After removing the argument "An umbrella provides the best," you are left with "defense against wet weather," which the writer assumes is a desirable thing.
Write the assumption in the thesis in your own words: "The thesis assumes everyone would want to stay dry in wet weather."
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