Rule of Assumption/Explanation

Proof Rule
Let the Rule of Assumption be invoked.

It does not matter whether the assumption is true -- all we are concerned about is making sure that any conclusion based on the assumptions made is as the result of a valid argument.

The introduction of an assumption $\phi$ into an argument by means of the Rule of Assumption can be interpreted in natural language as:
 * "Suppose it were true that $\phi$"

or:
 * "What if $\phi$ were true?"