Just as the question of whether material things exist objectively
(mind-independently) or only subjectively (dependent on a mind or
minds) is an unresolved question in ontology, so is the question of
whether ideas exist objectively or subjectively. During the Middle
Ages and early modern age, the latter question was as controversial as
the former.

Today there is little debate; it is commonly assumed that ideas have a
purely subjective existence. That conclusion does not seem to be a
product of any resolution of the question, but purely of forgetting
the question. One reason for that has been the rreplacement of
religion with science as a source of worldviews; science does not
assign any explanatory role to ideas.

To those within the religious paradigm, though, the question is still
current, and still as controversial. In that light comes this
treatment of the subject from the article, "Idealism," in the Catholic
Encyclopedia online. http://www.newadvent.org/cathen/07634a.htm

What follows is quoted entirely from that article. I have, though,
made liberal cuts in the material [indicated with square brackets],
and have divided the quoted material using my own headings [ditto].

[Objective Idealism]

Platonism is the oldest form of idealism, and Plato himself the
progenitor of idealists. It is usual to place in contrast Plato's
idealism and Aristotle's realism;[...] but for both, what lies deepest
down in their philosophy is the conviction that the first and highest
principle of all things is the one perfect spiritual Being which they
call God, and to which they lead back, by means of intermediate
principles--essence and form, purpose and law--the multifarious
individual beings of the visible world. In this sense idealism is
dualism, i.e. the doctrine of a higher spiritual principle over
against that which is lower and material; and this doctrine again is
clearly opposed to the monism which would derive the higher and the
lower alike out of one and the same All-being.[...] By means of its
principles, idealism maintains the distinctness of God and the world,
of the absolute and finite, yet holds them together in unity; it
adjusts the relations between reality and knowledge, by ascribing to
things dimension, form, purpose, value, law, at the same time securing
forethought the requisite certainty and validity; it establishes
objective truth in the things that are known and subjective truth in
the mind that knows them. In this sense the Schoolmen teach that forma
dat esse et distingui, i.e. the principle which formally constitutes
the object, likewise, in the act of cognition, informs the mind.[...]

In this sense St. Augustine developed the Platonic teaching, and in
his philosophy is idealism in the genuine meaning of the term. From
him comes the definition of ideas which Christian philosophy has since
retained: "Ideas are certain original forms of things, their
archetypes, permanent and incommunicable, which are contained in the
Divine intelligence. And though they neither begin to be nor cease,
yet upon them are patterned the manifold things of the world that come
into being and pass away. Upon these ideas only the rational soul can
fix its gaze, endowed as it is with the faculty which is its peculiar
excellence, i.e. mind and reason [mente ac ratione], a power, as it
were, of intellectual vision; and for such intuition that soul only is
qualified which is pure and holy, i.e., whose eye is normal, clear,
and well adjusted to the things which it would fain behold" (De
diversis quaest., Q. xlvi, in P.L., XL, 30).

This line of thought the Scholastics adopted, developing it in their
treatises as ideology. Their theory is described not as idealism, but
as realism; but [...] this means rather that the ideal principles
possess real validity, that as ideas they subsist in the Divine mind
before the things corresponding to them are called into existence,
while, as forms and essences, they really exist in nature and are not
really products of our thinking.

[Subjective Idealism]

In this last-named sense, i.e., as subjective constructions, ideas had
long before been regarded by the philosophers of antiquity and
especially by the Stoics, who held that ideas are nothing else than
mental representation. This erroneous and misleading view appeared
during the Middle Ages in the guise of nominalism, a designation given
to the system whose adherents claimed that our concepts are mere names
(nomina), which have as their counterparts in the world of reality
individual things, but not forms or essences or purposes. This
opinion, which robs both science and moral principles of their
universal validity, and which paves the way for Materialism and
agnosticism, [...] had its champions and propagators, notably William
of Occam. For the untrained mind it was easier to consider individual
things as the only realities and to regard forms and essences as
purely mental products.

So it came to pass that the word idea in various languages took on
more and more the meaning of "representation", "mental image", and the
like. Hence too, there was gradually introduced the terminology which
we find in the writings of Berkeley, and according to which idealism
is the doctrine that ascribes reality to our ideas, i.e. our
representations, but denies the reality of the physical world. This
sort of idealism [...] does away with the reality of ideal principles
by confining them exclusively to the thinking subject; it is a
spurious idealism which deserves rather the name
"phenomenalism" (phenomenon, "appearance", as opposed to noumenon,
"the object of thought").

The doctrine of Descartes has also per nefas been called idealism. It
is true that Cartesianism is in line with the genuine idealism of the
earlier schools, inasmuch as it postulates God, thought, and spatial
reality. But, on the other hand, this system too employs idea only in
a subjective signification[....] According to the theory of Leibniz,
which has also been regarded as idealistic, our mind constructs from
its own resources (de son propre fond) its scheme of the world; but ,
thanks to a pre-established harmony (harmonie préétablie), it accords
with reality. This view, however, furnishes no solution for the
epistemological problem.

Kant claims that his critical philosophy is both a "transcendental
idealism" and an "empirical realism"; but he declares ideas are
"illusions of reason", and such ideal principles as cause and purpose
are simply devices of thought which can be employed only in reference
to phenomena. Fichte took Kant as his starting-point but finally rose
above the level of subjectivism and posited a principle of reality,
the absolute Ego. Hegel's doctrine can be termed idealism so far as it
seeks the highest principle in the absolute idea, which finds its self-
realization in form, concept, etc.- a view which amounts virtually to
monism. The various offshoots of Kantian philosophy are incorrectly
regarded as developments of idealism; it is more accurate to describe
them as "illusionism" or "Solipsism", since they entirely sweep away
objective reality.[...]

Publication information
Written by Otto Willmann. Transcribed by Peter S. Zehr and Patrick C.
Swain.
The Catholic Encyclopedia, Volume VII. Published 1910. New York:
Robert Appleton Company. Nihil Obstat, June 1, 1910. Remy Lafort,
S.T.D., Censor. Imprimatur. +John Cardinal Farley, Archbishop of New
York
http://www.newadvent.org/cathen/07634a.htm