In a world where magic exists, magic exists. We can imagine a plan for making one, given uploading or better brain-perception interfaces and much better computing hardware. So it can exist both in principle and in practice. It might be that there is not much evidence for magic to shift expectation away from matter-is-dumb-stuff, as in our world, but even that doesn't necessarily rule it out.
Before evolution was figured out, unobserved living and perhaps thinking causes of life might have seemed a possibility, a reasonable expectation of nonvanishing probability. Future capability for making simulations with magic increase probability that a given medieval-like society is inhabiting one, although that argument probably wouldn't occur to its inhabitants, and capability isn't sufficient without motive, which seems tenuous. In any case, for practical purposes of building a technological civilization it shouldn't have mattered for our world, as not all probability went there and even in a magical world a technological civilization might be possible if there is no systematic/purposeful supernatural interference against that very outcome.
Our present certainty in the absence of magic is based on overwhelming evidence from the last few centuries of science and engineering, evidence about our world. Some of this evidence acts indirectly, for example once life was explained by evolution and no other settled evidence of supernatural (i.e. minds other than human or animal minds) or processes that could originate it turned up, there was no reason to expect anything further. Noticing biases of projecting mind-like properties on other things and social processes that create unfounded status quo belief systems should also retract some of the belief in other things having mind-like properties. This is of particular interest in modern times when it's about the only reason that a smart person can still manage to hold this misconception.
But for an inhabitant of a magical world, there is evidence of magic, sometimes overwhelming evidence, and there is no contradiction with our world having no magic.
Not sure we understand each other. It is easy to imagine worlds where the paranormal exists. But it is something deeper - it is about inrreducibly mental phenomena. To stick to my example, a potion or fountain of healing depends on the mental model of what changes in the body do human minds consider beneficial. It is conceivable as something designed by a human mind e.g. nanomachines, but as a naturally, paranormal-naturally, supernatural-naturally occuring phenomenon it is simply a logical contradiction in all logically conceivable worlds, unless a non-hu...
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