Belief in the “supernatural ” belongs to a “bygone era” (along with the belief in ghosts or that ideas exist outside of mind.) The very act of entertaining the existence of ghosts reflects the continued influence of pre-scientific, mythological thinking on the present; and despite the privileged position reason, logic and science currently occupy, Western culture appears incapable of entirely shedding its ancient skin, e.g. we still call our galaxy the Milky Way even though no one believes in the existence of the goddess Hera; while continuing to entertain the idea of mind-body dualism despite advances in neuroscience which quite satisfactorily describe consciousness—“ideas” if you will—as an emergent quality born out of the complex physical workings of the human brain. There is still a minority of neuroscientists who entertain the notion consciousness implies that the sum of the brain’s parts alone do not satisfactorily explain consciousness; nevertheless, this is a minority position and the neuroscience community appears to have made peace with the fact a physical explanation agrees with observation. Or to invoke Ockham’s Razor the mechanistic explanation definitely makes far fewer assumptions than the idealist explanation does.
So, no brain? No ideas. Yet, an argument can be made ideas potentially exist, i.e. ‘truth’ and ‘beauty’ do not exist “out there” as in some sort of Platonic form; however, it is quite reasonable to suppose—and I’ve heard various philosophers and scientists seriously consider this hypothesis—that once an appropriate mechanism evolves (like the brain, for example) consciousness and then ideas inevitably follow. So, in a sense, ideas exist independent of mind as something potential rather than actual; they just need a host in the same sense a bow needs an archer to pull the string.
Why do people continue to entertain beliefs in things like soul, spirit, idealism, mind-body dualism though? There are a combination of factors but I would appeal first and foremost to Thomas Paine’s explanation, i.e. the long habit of thinking a thing true gives it the superficial appearance of being right.
People have thought these things exist or are true for so long the culture has literally succumbed to a sort of organic or inherited “argument from antiquity”. The reason you appear to even entertain idealism (and by extension mind-body dualism) is Western culture–to which you belong–was shaped considerably by Hellenistic thought. From the Greeks we inherited some useful ways of looking at the world and some not so useful. For example, from the rationalist Thales we inherit the idea that we can explain what happens in the natural world (like a volcano erupting or lightning striking) by appealing to natural causes (or mechanisms) rather explaining these things by saying Hephaestus or Zeus are angry. We also inherited the assumption that souls, spirit and mind exist independent of the body. Plato, as I mentioned previously, even went so far as to claim, as you’ve entertained, ideas exist “out there” objectively and that the so-called “mind’s eye” perceived them. The problem with idealism, souls, gods, mind-eyes, etc. is there’s no reason (no evidence) to suppose any of it reflects the way the world actually works. Gods, etc. were all constructed from common sense deductions—based on the assumptions of the time—that offered a pseudo-scientific explanation satisfying the ancient Greeks. Nobody told the inheritors of Greeks (us), however, that not only could we drop belief in gods but also assumptions about souls and objective ideas, as well.