Inferences and Assumptions

When glancing through LSAT questions, you will frequently see the words
inference and assumption. Let us take a moment to define the meaning of each
term in the context of LSAT argumentation.
Most people have come to believe that the word inference means probably true
or likely to be true. Indeed, in common usage infer is often used in the same
manner as imply. On the LSAT these uses are incorrect. In logic, an inference
can be defined as something that must be true. Thus, if you are asked to
identify an inference of the argument, you must find an item that must be true
based on the information presented in the argument.
Earlier we discussed assumptions in the context of commonsense assumptions
that you can bring into each problem. In argumentation, an assumption is
simply the same as an unstated premise—what must be true in order for the
argument to be true. Assumptions can often have a great effect on the validity
of the argument.
Separating an inference from an assumption can be difficult because the
definition of each refers to what “must be true.” The difference is simple: an
inference is what follows from an argument (in other words, a conclusion)
whereas an assumption is what is taken for granted while making an argument.
In one sense, an assumption occurs “before” the argument, that is, while the
argument is being made. An inference is made “after” the argument is
complete, and follows from the argument. Both concepts will be discussed in
more detail in later chapters, but for the time being you should note that all
authors make assumptions when creating their arguments, and all arguments
have inferences that can be derived from the argument.