How would you respond to a person who tells you not to believe scientific theory because theories are only possible for explanations that are not packed by evidence

I would explain to him ... calmly, coolly, and patiently ... that

the 'evidence' is always there before the theory.

Real scientists do not gaze into their tea leaves, dream up theories

that sound cool, and then go out to find evidence that supports the

theory. That is not the way that science works, and any individual

who operates that way is simply never heard from again.

In real science, everybody keeps his eyes wide open, observing

everything that goes on around him, measuring it, photographing it,

keeping records of how often it happens, in what situations and

under what conditions, and what came before and after it.

Eventually, he tries to put together an explanation of what causes

the things that he has seen, and how they work. The explanation

must fit what he has seen. That explanation is his theory, created

to explain what has already been seen.

And if the explanation is any good, it predicts things that have

not been seen yet, but he and other scientists can look for them.

If they look and do find them, then those findings support the

theory. If they look and find things completely opposite to what

the theory predicts, then two things could happen: Either the

theory is adjusted to account for the new observations, or else

the theory is completely rejected.

So, your friend has stated that theories are only possible when

not backed by evidence. When he makes that statement, he

reveals that he knows not whereof he speaks, for the truth is

exactly the opposite: Theories arise out of evidence, and where

there is no evidence, there's no need for a theory, and no theory

is possible.