Rationalism and idealism contradict each other

The argument

Socrates, Plato, Berkeley, Hume, Kant and Schopenhauer are all idealist-rationalist philosophers. In one form or another, all six deny the independent existence of matter. All six deny that matter is primary and mind is secondary. All six maintain that what we call material objects and the material world exist only in being perceived. In this sense, all are idealists, not materialists. This might seem to constitute a contradiction. If matter and material objects exist only in being perceived, then how can there be any objective knowledge of the world? And if there can be no objective knowledge of an independently existing material world, then isn't all human knowledge merely subjective? If a philosopher condemns all human knowledge to being purely subjective, then how can such a philosopher be classified as rational or a rationalist? Clearly, there is nothing rational about denying that there are independently existing material objects. If a philosopher maintains that objective knowledge or universal truths are impossible, then a philosopher who holds that position cannot be classified as a Rationalist. Yet the Rationalist-Idealist philosophers all claim that they can circumvent the seeming absurdities and contradictions. If material objects are nothing more than ideas in a mind, nothing more than subjective perceptions that have no existence apart from perception, then how is objectivity possible? Either our ideas and perceptions depend on matter and must conform to matter if they are to be objective, or matter depends on our ideas and perceptions and are perfectly subjective. So which is primary: ideas of objects or material objects; mind or matter?

Premises

Counter-arguments

The argument's roster does not survive inspection, and the roster is what the case rests on. Hume is the standard opponent of rationalism, not an example of it — his account of causation and of the limits of reason is the empiricist challenge rationalists set out to answer. Berkeley is likewise classed as an empiricist, his immaterialism developed out of Locke rather than against him. Kant explicitly rejected both dogmatic rationalism and Berkeleyan idealism, and included in the Critique of Pure Reason a Refutation of Idealism arguing that inner experience presupposes objects outside us; his position pairs transcendental idealism with empirical realism. And Plato's Forms are the opposite of mind-dependent — they exist independently of any perceiver, which is why his position is usually called realism about universals. That exposes an equivocation running through the argument. 'Idealism' names at least two different doctrines: the claim that reality is mental, and Platonic realism about abstract objects. They share a label and little else, and the argument slides between them. The central inference also fails on the terms of the philosophers named. Berkeley held that objects persist because they are sustained in God's perception, which secures objectivity independent of any individual mind. Kant held that the forms of intuition and the categories are universal across rational subjects, which is precisely how he thought objective and necessary knowledge is possible. Mind-dependence and subjectivism are not the same claim.

Rejecting the premises

[Rejecting P1] Hume and Berkeley are standardly classed as empiricists rather than rationalists, Kant explicitly refuted Berkeleyan idealism while defending empirical realism, and Plato's Forms exist independently of any perceiver. [Rejecting P2] Mind-dependence does not entail subjectivism: Berkeley grounded the persistence of objects in divine perception and Kant grounded objectivity in structures universal to all rational subjects.