Assuming 1) is a true premise and 2) ??? is some black box function, it's hard to reason in any way that 3) is not true to some extent. When you change the input, you would expect the output to change unless you can somehow argue that ??? is f(x) = C. You can say step 2) is egregious handwaving, but as long as you cannot prove f(x) is a constant, 3) is true.