How did ideas as conceived originally by Plato as the 'looks' of beings as beings degenerate into becoming, with Descartes, representations inside consciousness and then further today into 'ideas' in the head? The last are then finally (apparently) reduced to neural configurations of the material brain by today's neuroscience that seems, once and for all, to have put the mind-body problem effectively to rest.
This degeneration of the mind runs parallel to another as a consequence of Parmenides' warning not having been heeded. Namely, he warned not to separate thinking from being:
τὸ γὰρ αὐτό νοεῖν ἐστίν τε καὶ εἶναι. Diels/Krantz Fragment 3
"For thinking and being belong together." *
This has been taken as a seminal formulation of so-called 'idealism' as a philosophical 'position' that fights to maintain its position against other positions such as (various varieties of) 'realism' and 'materialism', whereby the conception of the idea itself has been thoroughly misunderstood. Namely ideas are understood as being 'about' beings, i.e. ontic, rather than their being ontological interpretations of the being of beings, i.e. of their respective modes of being. Misinterpreted ontically, Fragment 3 seems to be saying some kind of magical formula: reality conforms to the way you think it is, with the consequence that, if you change your mind, reality will change in line with your thoughts.
But the idea is ontological, conceptualizing as it does a mode of being of beings through which the mind understands reality, i.e. the world. The idea in this sense is not individual, but shared in an historical time. The shared mind of a given time, its Zeit-Geist, is 'built' from the building-blocks of the ideas constituting in their interconnection the shared, inescapable understanding holding sway in an historical age.
By ignoring and misinterpreting both Parmenides' warning and Plato's ontological conception of the idea, the Western mind has gone 'pro-gressively' downhill i) to split thinking from being, with subjectivity on the inside and objectivity on the outside and ii) to think thinking itself only ontically, with scarcely a trace that ideas in the philosophical sense are ontological.
The closure and suppression of the ontological difference can be blamed especially on Anglo-American philosophy in the guises of British empiricism, American pragmatism, analytic philosophy, etc. The closure is reflected inversely in the rise of positivism and the establishment of the reign of materially-, evidence-based scientific thinking. For this way of thinking, the evidence of the phenomena themselves is ignored in favour of constructing theoretical models that aim at somehow or other causally explaining, and thus predicting, various kinds of movements in the world.
All the more reason to go back to scratch to think again.
* For further alternative translations of Fragment 3, cf. my Parmenides article.
Further reading: 'Out of your mind? Parmenides' message'
On Human Temporality: Recasting Whoness Da Capo (De Gruyter 2024 in press)