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.

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